Laser light source device and image display device

ABSTRACT

The present invention concerns a laser light source device capable of multiwavelength oscillation. This laser light source device is provided with a laser light source; a laser cavity including a fiber, a first fiber grating provided at a side of the fiber toward the laser light source and having a plurality of reflection peaks, and a second fiber grating provided at a light emission end of the fiber and having a plurality of reflection peaks: a wavelength converter for converting a fundamental wave emitted from the laser cavity into a harmonic wave; a reflection wavelength varying unit capable of shifting the reflection wavelengths of the reflection peaks of the second fiber grating; and a controller for controlling phase matching conditions of the wavelength converter. Intervals between adjacent reflection peaks of the first fiber grating are different from those between adjacent reflection peaks of the second fiber grating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser light source device forobtaining a stable and high-output laser light by combining a fiberlaser and a wavelength conversion element, and an image display deviceusing such a laser light source device.

2. Description of the Background Art

High-output visible light sources having strong monochromaticity arenecessary in realizing large-scale displays, high-luminance displays andthe like. Out of three primary colors of red, green and blue, a redhigh-output semiconductor laser used in a DVD recorder or the like canbe utilized as a small-scale light source having high productivity for ared color. However, for a green or blue light source, realization bymeans of a semiconductor laser or the like is difficult and there is ademand for a small-scale light source having high productivity.

For such a light source, a wavelength conversion device constructed bycombining a fiber laser and a wavelength conversion element is realizedas a low-output visible light source. Green and blue small-scale lightsources are well known which use a semiconductor laser as an excitationlight source for exciting the fiber laser and a nonlinear opticalcrystal as the wavelength conversion element.

On a laser display utilizing such high-output laser light sources, vividimages with high color purity can be displayed by utilizing laser lightsources having suitable wavelengths since lights of the respective red,green and blue light sources are monochromatic lights. Such laser lightsources enable the miniaturization of light sources by the use of lasersand the miniaturization of optical systems because lights can be easilyfocused, whereby palmtop image display devices can be realized.

On the other hand, the use of a laser having high coherency causescoherence noise called speckle noise to be formed in a displayed image.Accordingly, there have been proposed methods for removing speckle noiseaccording to which the speckle noise is reduced by differentiating lightpaths depending on polarization directions using a prism (JapaneseUnexamined Patent Publication No. 2004-151133); the wavefront of a lightirradiated to a screen is made random by swinging an optical componentto change a light path of a light source (Japanese Unexamined PatentPublication No. 2004-138669); a spectrum is caused to have a sidebandusing a light modulator to broaden the apparent light spectrum (JapaneseUnexamined Patent Publication No. H09-121069), an oscillation wavelengthis operated using the injection seeding technology to a solid-statelaser (Japanese Unexamined Patent Publication No. H10-294517); and asemiconductor laser having a plurality of wavelengths is used as amodule (Japanese Unexamined Patent Publication No. 2004-144794).

On the other hand, there have been proposed a fiber laser using a Ramanfiber (WO 01/54238) and a semiconductor laser using a reflection elementcalled a sampled grating which is a diffraction grating having aplurality of reflection wavelengths to realize a semiconductor lasercapable of simultaneous multiwavelength oscillation (Japanese Patent No.3689483). The construction of the fiber laser disclosed in WO 01/54238is shown in FIG. 26A. This laser includes a sampled grating 2001, aRaman fiber 2002 and a broadband dielectric mirror 2003. By exciting theRaman fiber 2002 with an excitation light 2004, a light of λ1 isgenerated by the resonance between one reflection peak of the sampledgrating 2001 and the broadband dielectric mirror 2003. Using this lightof λ1 as an excitation light, a light of λ2 is generated by theresonance between another reflection peak of the sampled grating 2001and the broadband dielectric mirror 2003. In this way, lights of λ3, λ4are successively generated and a light 2005 of λ5 is finally extracted.

The construction disclosed in Japanese Patent No. 3689483 is shown inFIG. 26B. The combined construction of a Raman fiber and a sampledgrating in FIG. 26B aims to generate an excitation light of a desiredwavelength by a Raman shift. An emission area 2007 and a reflection area2008 are formed on a semiconductor substrate 2006, wherein thereflection area 2008 has a sampled grating structure so as to be capableof simultaneous oscillation at a plurality of wavelengths. Control means(refractive index changing electrode) 2009 for changing the refractiveindex of the reflection area 2008 is provided in the reflection area2008, so that the wavelength of the reflection area 2008 can be shifted.

U.S. Pat. No. 6,432,736 discloses an example of a semiconductor laserarray in which two reflection areas having a sampled grating structureare provided to switch an oscillation wavelength. Further, U.S. Pat. No.6,597,711 discloses an example in which the construction of U.S. Pat.No. 6,432,736 is applied to a fiber laser.

However, if the aforementioned prior arts are applied to a method forreducing speckle noise by using broadband light sources or light sourceshaving a plurality of oscillation wavelengths, there have been problemsof a higher cost, an enlarged device size and the like since a pluralityof laser light sources are required. The semiconductor laser and thefiber laser light source using the sampled grating cannot obtain as highoutputs as can be used as light sources for laser displays and, inaddition, determine the oscillation wavelength by controlling the tworeflection areas. Therefore, there has been a problem of complicating acontrol method for simultaneously controlling the oscillation wavelengthand the laser output. There is another problem that a wavelength changeby the control of the reflection areas is sensitive to externaltemperature. For the example of the fiber laser light source, a largestress needs to be applied to the fiber grating in order to obtain alarge variable range, making the breakage of the fiber possible,wherefore there is a problem in the reliability of the fiber. There isan additional problem that the wavelength dependency of a gain of alaser medium triggers an oscillation output change in the case ofswitching the wavelength.

On the other hand, the aforementioned laser light source having aplurality of oscillation wavelengths has started being used in themedical field. Laser lights of different wavelengths are neededdepending on treatments in the medical field. The wavelengths of laserlights particularly used in eye clinics are in the neighborhood of 530nm, in the neighborhood of 600 nm and in the neighborhood of 1 μm. Laserlights in the neighborhood of 530 nm are used for the retinalcoagulation of eyes, those in the neighborhood of 600 nm for thestoppage of fundal hemorrhage, and those in the neighborhood of 1 μm forcataract surgeries. Developments on laser light sources used for suchophthalmic treatments are being advanced at present and, particularly,there has been a need for a laser application device capable ofobtaining laser lights of many wavelengths so as to cope with manytreatments by one laser light source. Japanese Unexamined PatentPublication No. 2006-122081 discloses a laser device capable ofmultiwavelength oscillation by shifting an oscillation wavelength towarda longer wavelength side using a Raman fiber. Besides, lasers and thelike have been developed which realize multiwavelength oscillation byutilizing a plurality of fluorescence peaks of a solid-state laser.

However, the above laser device capable of multiwavelength oscillationusing the Raman fiber cannot simultaneously generate lights of twowavelengths. Further, since the gain of the Raman fiber laser isgreatest at a wavelength of 1000 to 1100 nm, in the case of generating alight of 1100 to 1200 nm necessary for an orange light, a light having abroad emission spectrum of 1000 to 1100 nm is also generated. This hascaused a problem of breaking a laser oscillator by the pulseoscillation. On the other hand, in the aforementioned laser capable ofmultiwavelength oscillation using the solid-state laser, the opticalsystems need to be switched for each oscillation wavelength, which hascaused a problem of difficulty to switch to a desired wavelength in amoment. Further, since the property of the laser light largely differsat each oscillation wavelength, there has been a problem that obtainedmaximum outputs cannot be constant.

Further, in the construction of the conventional laser light source formedical use, a laser light generated by the laser oscillator has beenpropagated to a surgical handpiece by means of a hollow fiber or thelike after having a fundamental wave thereof wavelength-converted into avisible light by a wavelength conversion element. However, about 30% ofthe visible light is lost to reduce propagation efficiency due to thecoupling loss of the visible light from the laser oscillator to thehollow fiber and a propagation loss in the fiber. There has been also aproblem that the handpiece is difficult to handle due to the handling ofthe fiber.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a highly reliably laserlight source capable of generating laser lights while switching awavelength among a plurality of wavelengths and having as large a lightoutput as W-class.

One aspect of the present invention is directed to a laser light sourcedevice, comprising a laser light source for emitting an excitationlight; a laser cavity including a fiber which contains a laser-activesubstance and on which the excitation light from the laser light sourceis incident, a first fiber grating provided at a side of the fibertoward the laser light source and having a plurality of reflectionpeaks, and a second fiber grating provided at a light emission end ofthe fiber and having a plurality of reflection peaks; a wavelengthconverter for converting a fundamental wave emitted from the lasercavity into a harmonic wave; a reflection wavelength varying unitcapable of shifting the reflection wavelengths of the reflection peaksof the second fiber grating; and a controller for controlling theoscillation wavelength of the laser cavity by means of the reflectionwavelength varying unit and controlling a phase matching condition ofthe wavelength converter, intervals between adjacent reflection peaks ofthe first fiber grating being different from those between adjacentreflection peaks of the second fiber grating.

In the above laser light source device, each of the first and secondfiber gratings has a plurality of reflection peaks, and the intervalsbetween adjacent reflection peaks of the first fiber grating are set tobe different from those between adjacent reflection peaks of the secondfiber grating. Therefore, the oscillation wavelength of the laser cavitycan be switched by shifting only the reflection wavelengths of thereflection peaks of the second fiber grating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic construction of a laser lightsource according to a first embodiment of the invention,

FIG. 2A is a graph showing a reflection spectrum of a first fibergrating,

FIG. 2B is a graph showing a reflection spectrum of a second fibergrating,

FIGS. 3A to 3C are graphs showing an operation of selecting thewavelength of the laser light source according to the first embodimentof the invention,

FIG. 4 is a graph showing a relationship between the oscillationwavelength and loss of a Yb doped fiber,

FIG. 5 is a graph showing a relationship between the reflectionwavelength and reflectance of the second fiber grating,

FIG. 6A is a schematic diagram showing one construction of a wavelengthconverter,

FIG. 6B is a schematic diagram showing another construction of thewavelength converter,

FIG. 7 is a schematic diagram showing still another construction of thewavelength converter,

FIG. 8 is a schematic diagram showing further another construction ofthe wavelength converter,

FIG. 9A is a diagram showing one structure of polarization reversalperiods of a nonlinear optical crystal constituting a wavelengthconversion element,

FIG. 9B is a diagram showing another structure of the polarizationreversal periods of the nonlinear optical crystal constituting thewavelength conversion element,

FIG. 9C is a diagram showing still another structure of the polarizationreversal periods of the nonlinear optical crystal constituting thewavelength conversion element,

FIG. 10 is a diagram showing an input/output characteristic of afundamental wave of an excitation laser,

FIG. 11 is a diagram showing a schematic construction of atwo-dimensional image display device according to a second embodiment ofthe invention,

FIG. 12 is a chromaticity diagram showing a relationship of thewavelength and color reproduction range of a green light,

FIG. 13 is a diagram showing a schematic construction of a laser lightsource according to a third embodiment of the invention,

FIG. 14 is a diagram showing a schematic construction of a laserfluorescence microscope according to a fourth embodiment of theinvention,

FIG. 15A is a graph showing an oscillation spectrum of a fiber laser atthe time of generating an ASE,

FIG. 15B is a graph showing an oscillation spectrum of the fiber laserat the time of devising a countermeasure against the ASE,

FIG. 16 is a diagram showing a schematic construction of a laser lightsource device according to a fifth embodiment of the invention,

FIG. 17A is a graph showing a change of a characteristic curve of aphase matching wavelength of a wavelength conversion element,

FIG. 17B is a graph showing another change of the characteristic curveof the phase matching wavelength of the wavelength conversion element,

FIG. 18 is a flow chart showing the procedure of a fiber gratingtemperature controlling process,

FIG. 19A is a diagram showing a schematic construction of a laser lightirradiator,

FIG. 19B is a diagram showing another schematic construction of thelaser light irradiator,

FIG. 19C is a diagram showing still another schematic construction ofthe laser light irradiator,

FIG. 20A is a graph showing a relationship between oscillationwavelengths of a fiber laser and reflection wavelength bands of a fibergrating,

FIG. 20B is a graph showing another relationship between oscillationwavelengths of the fiber laser and reflection wavelength bands of thefiber grating,

FIG. 21 is a flow chart showing the procedure of a light irradiationprocess of the laser light irradiator,

FIG. 22 is a diagram showing a schematic construction in which a probeis attached to a light irradiation surface of the laser lightirradiator,

FIG. 23 is a graph showing a relationship between the fundamental waveoutput and oscillation wavelength bandwidth of the fiber laser,

FIG. 24 is a graph showing a relationship between the wavelengthbandwidth of fundamental wave and conversion efficiency of the fiberlaser,

FIG. 25 is a graph showing another relationship between the fundamentalwave output and oscillation wavelength bandwidth of the fiber laser,

FIG. 26A is a diagram showing a schematic construction of a conventionalfiber laser, and

FIG. 26B is a section showing a schematic construction of a conventionalsemiconductor laser.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, embodiments of the present invention are described withreference to the accompanying drawings. It should be noted that the sameparts are identified by the same reference numerals and those assignedwith the same reference numerals in the drawings may not be repeatedlydescribed in some cases.

First Embodiment

FIG. 1 is a diagram showing a schematic construction of a laser lightsource according to a first embodiment of the present invention. In FIG.1, a laser light source 21 of this embodiment is provided with a fiberlaser 22, a wavelength converter including a wavelength conversionelement 25 for converting a fundamental wave 53 emitted from the fiberlaser 22 into a harmonic wave 24, and a controller 34.

The fiber laser 22 includes a fiber 26 containing a laser-activesubstance, an excitation laser 28 for emitting an excitation light 27 tothe fiber 26 via a fiber 32, and a first fiber grating 29 and a secondfiber grating 30 formed at the opposite ends of the fiber 26 andconstructing a laser cavity together with the fiber 26.

The first fiber grating 29 has a plurality of broadband reflection peaksand the bandwidths of the respective reflection peaks are 0.5 to 3 nm.The second fiber grating 30 also has a plurality of broadband reflectionpeaks and the bandwidths of the respective reflection peaks are 0.2 nmor shorter. The cavity of the fiber laser 22 amplifies and emits thefundamental wave 53 using the reflection peaks selected one each fromthe first and second fiber gratings 29, 30 as two reflecting surfaces.The bandwidth of each reflection peak of the second fiber grating 30 ispreferably equal to or below 0.2 nm, more preferably equal to or below0.15 nm. This is because the wavelength conversion element 25 can moreefficiently perform wavelength conversion as the bandwidth of the secondfiber grating 30 becomes narrower to satisfy a wavelength permissiblerange at the time of the wavelength conversion by the wavelengthconversion element 25. Since the wavelength band of the oscillatedfundamental wave 53 needs to be narrow in such a wavelength conversionapplication technology, the wavelength band of the second fiber grating30 needs to be narrowed. Accordingly, a rough control can be set for thesecond fiber grating 30 at a narrower band side by setting the bandwidthof the first fiber grating 20 to about the tenfold of that of the secondfiber grating 30, i.e. by setting the bandwidth of the first fibergrating 29 to 0.5 to 3 nm, more preferably to 0.5 to 2 nm. The bandwidthof the first fiber grating 29 is preferably set to 0.5 to 2 nm sinceripples seen in the top shape of the band become larger if the bandwidthof the first fiber grating 29 is excessively widened.

The second fiber grating 30 is arranged on a reflection wavelengthvarying unit 33. The reflection wavelength varying unit 33 has a stressapplying mechanism for applying a tensile stress to the second fibergrating 30. By applying a tensile stress to the second fiber grating 30,the reflection wavelength of the second fiber grating 30 is shifted,thereby changing the oscillation wavelength of the fiber laser 22. Oneend of the second fiber grating 30 is fixed on a base and the other endthereof is fixed on a one-axis stage driven by a pulse motor. A movabledirection of the stage and a light propagation direction of the secondfiber grating 30 are parallel, and a tensile stress can be applied tothe second fiber grating 30 by the rotation of the pulse motor. Althoughthe reflection wavelength varying unit 33 is provided with the stressapplying mechanism here, this embodiment is not limited thereto. Forexample, the reflection wavelength varying unit 33 may be provided witha temperature controlling mechanism using a Peltier element and thereflection wavelength may be shifted by controlling the temperature ofthe second fiber grating 30.

Next, the respective plurality of reflection peaks of each of the firstand second fiber gratings 29, 30 are specifically described. Areflection spectrum of the first fiber grating 29 is shown in FIG. 2Aand that of the second fiber grating 30 is shown in FIG. 2B. In FIG. 2A,the first fiber grating 29 has three center wavelengths λ1, λ2 and λ3and a reflectance of 98% or higher, i.e. serves as a high-reflect (HR)mirror. On the other hand, in FIG. 2B, the second fiber grating 30 hasthree center wavelengths λ4, λ5 and λ6 and a reflectance of about 12%.The reflectance of about 12% is necessary because the second fibergrating 30 functions to feed a part of an oscillation light to the fiber26. In FIGS. 2A and 2B, the first fiber grating 29 has a bandwidth of0.5 nm and such a property that the center wavelengths are spaced apartat intervals of 5 nm. On the other hand, the second fiber grating 30 hasa bandwidth of 0.06 nm and such a property that the center wavelengthsare spaced apart at intervals of 4.5 nm. The intervals between therespective center wavelengths of the first and second fiber gratings 29,30 are suitably set based on an oscillation wavelength required for thefiber laser 22. In FIGS. 2A and 2B, the first and second fiber gratings29, 30 are kept at room temperature (25° C.) and λ1=λ4 if no tensilestress is applied to the second fiber grating 30.

At this time, if two semiconductor lasers having a threshold current of0.45 A, a maximum output of 7 W and a wavelength of 915 nm were used asthe excitation laser 28, the fundamental wave 53 having an output of 7.2W could be realized. By using an excitation wavelength of 915 nm, thefundamental wave 53 can be stably outputted by a simple coolingmechanism using a cooling fan and a blast fan.

Next, an operation of selecting the wavelength of the laser light sourceaccording to this embodiment is descried. Here is described the casewhere the first and second fiber gratings 29, having the reflectionpeaks shown in FIGS. 2A and 2B are used. FIGS. 3A to 3C are graphsshowing the operation of selecting the wavelength of the laser lightsource according to this embodiment, wherein FIG. 3A shows a state atroom temperature where no tensile stress is applied to the second fibergrating 30, FIG. 3B shows a state at room temperature where thereflection wavelength of the second fiber grating 30 is shifted only by0.5 nm by applying a tensile stress to the second fiber grating 30, andFIG. 3C shows a state at room temperature where a tensile stress isfurther applied to the second fiber grating 30 to shift the reflectionwavelength of the second fiber grating 30 by 0.5 nm.

First, in FIG. 3A, the center wavelength λ1 of the first fiber grating29 and the center wavelength λ4 of the second fiber grating 30 coincidein FIG. 3A. Thus, the fiber laser 22 oscillates at λ1 (=λ4).

Next, in FIG. 3B, the reflection wavelength of the second fiber grating30 is shifted through the application of a tensile stress by thereflection wavelength varying unit 33. As a result, the centerwavelength λ2 of the first fiber grating 29 and the center wavelength λ5of the second fiber grating 30 come to coincide. Thus, the fiber laser22 oscillates at λ2 (=λ5). In other words, the oscillation wavelength ofthe fiber laser 22 can be shifted by 5 nm only by shifting thereflection wavelength of the second fiber grating 30 by as small as 0.5nm.

In FIG. 3C, the reflection wavelength of the second fiber grating 30 isfurther shifted through the further application of a tensile stress bythe reflection wavelength varying unit 33. As a result, the centerwavelength λ3 of the first fiber grating 29 and the center wavelength λ6of the second fiber grating 30 come to coincide. Thus, the fiber laser22 oscillates at λ3 (=λ6) and the oscillation wavelength thereof isfurther shifted by 5 nm from the state shown in FIG. 3B.

In other words, the oscillation wavelength of the fiber laser 22 can beshifted by 10 nm by shifting the reflection wavelength of the secondfiber grating 30 by 1 nm. In this way, the oscillation wavelength of thefiber laser 22 can be switched by 5 nm each time, by a total of 10 nm,although these changes are discrete. A shift amount of the oscillationwavelength of the fiber laser 22 can be arbitrarily set depending on thedesigns of the first and second fiber gratings 29, 30.

As described above, by combining the first fiber grating 29 as areflector having a high reflectance and a wide bandwidth and the secondfiber grating 30 as a reflector having a low reflectance and a narrowbandwidth, it becomes unnecessary to control the oscillation wavelengthof the fiber laser 22 by finely adjusting both reflectors and it becomespossible to switch the oscillation wavelength only by controlling onereflector. In addition, with the conventional technology, theoscillation wavelength needs to be monitored based on the laser output,making it difficult to distinguish a change of the oscillationwavelength and a change of the laser output itself. In this embodiment,it is sufficient to control only the change of the laser output itself(automatic power control: APC) by eliminating the need to monitor theoscillation wavelength. Accordingly, the laser output can be more easilycontrolled. Further, with the conventional technology, there have beencases where the oscillation wavelength inadvertently switches becausethe reflection wavelength of the fiber grating is sensitive to anexternal temperature variation. In this embodiment, this problem canalso be prevented by making the bandwidth of the first fiber grating 29wider than that of the second fiber grating 30.

In the fiber laser used in the laser light source according to thisembodiment, an oscillation efficiency is known to differ at eachoscillation wavelength due to the absorption of the fiber by the dopingof Yb that is a laser-active substance. FIG. 4 shows a relationshipbetween the oscillation wavelength and loss of a Yb doped fiber. Asshown in FIG. 4, in an oscillation wavelength of 1045 to 1070 nm, thelonger the oscillation wavelength, the smaller the loss of the fiber.Thus, there is a tendency to increase the oscillation efficiency as theoscillation wavelength increases. Accordingly, a reduction in theoscillation efficiency needs to be prevented in the case of shorting theoscillation wavelength. In this embodiment, the reflectance of thesecond fiber grating 30 is increased for this reduction in theoscillation efficiency. Specifically, the reduction in the oscillationefficiency is prevented by improving the reflectance of the second fibergrating 30 to increase the oscillation light fed back to the fiber 26.The second fiber grating 30 having a plurality of reflection wavelengthscan change the lengths of the respective gratings corresponding to therespective reflection wavelengths to change the reflectances of therespective gratings. The reflectances of the respective gratingsincrease by the increased lengths of the respective gratings. In otherwords, the reflectances of the respective gratings are increased in thecase of short oscillation wavelengths (1045 nm in the case of thisembodiment) while being decreased in the case of longer oscillationwavelengths (1070 nm in the case of this embodiment), whereby thereduction in the oscillation efficiency can be suppressed. In this way,an increase in a drive current amount of the excitation laser 28 by APCcan be suppressed. In other words, lower power consumption can berealized. FIG. 5 shows an exemplary relationship between the reflectionwavelength and reflectance of the second fiber grating 30.

In FIG. 5, in the case of designing the fiber laser 22 so as tooscillate in a wavelength range X of from 1045 nm to 1070 nm at 5 nmintervals, the reduction in the oscillation efficiency of the fiberlaser 22 can be suppressed and an output variation of the fundamentalwave 53 caused by a variation of the oscillation wavelength of the fiberlaser 22 can be curbed to 5% or lower by setting the largest reflectionpeak of the second fiber grating 30 at 1045 nm and successivelyallotting reflection wavelengths in an decreasing order at 5 nmintervals.

Next, the wavelength converter 23 for generating the harmonic wave 24 ofthe fundamental wave 53 emitted from the fiber laser 22 is described. Asshown in FIG. 1, the wavelength converter 23 includes the wavelengthconversion element 25, a condenser lens 31, a beam splitter 8 and awavelength conversion element holder 35. Although the condenser lens 31is provided in the wavelength converter 23 in this embodiment, it may beprovided in the fiber laser 22.

When a laser light of the fundamental wave 53 is outputted by the fiberlaser 22, it is collected by the condenser lens 31 to be incident on thewavelength conversion element 25. When the fundamental wave 53 from thefiber laser 22 becomes an incident wave and is converted by thenonlinear optical effect of the wavelength conversion element 25, itbecomes the harmonic wave 24 having half the wavelength of thefundamental wave 53. A part of the converted harmonic wave 24 havingpassed through the beam splitter 8 is almost entirely emitted as anoutput light of the laser light source 21 while the remaining part isreflected by the beam splitter 8.

The part of the harmonic wave 24 reflected by the beam splitter 8 isreceived and converted into an electrical signal by a light receivingelement 37 to be used for the monitoring of the output light of thewavelength conversion element 25. The controller 34 adjusts a drivecurrent of the excitation laser 28 by the control of an excitation lasercurrent source 36 so that the intensity of this converted signal isincreased to obtain a desired output in the wavelength conversionelement 25. Then, the intensity of the excitation light 27 from theexcitation laser 28 is adjusted to adjust the output intensity of thefundamental wave 53 of the fiber laser 22, with the result that theoutput intensity of the laser light source 21 can be adjusted. In thisway, the intensity of the output of the laser light source 21 can bekept at a constant level and the so-called automatic power control (APC)is stably carried out. It should be noted that a light receiving elementmay be arranged at a side of the fiber 26 more outward than the secondfiber grating 30 in order to more accurately control the outputintensity of the laser light source 21 by the APC operation. In thisway, a slight leakage of the fundamental wave 53 without being reflectedby the second fiber grating 30 can be detected. By respectivelyestimating the overall intensities of the excitation light 27 and thefundamental wave 53 based on this detection data, the controller 34 canautomatically control the output intensity of the laser light source 21by adjusting the drive current of the excitation laser 28 by the controlof the excitation laser current source 36.

It should be noted that the detection of the fundamental wave 53 is notlimitedly made by the construction in which the light receiving elementis arranged outside the second fiber grating 30. Instead, a branchedlight of the fundamental wave 53 emitted from the second fiber grating30 may be detected. Alternatively, an output of the fundamental wave 53having passed through the first fiber grating 29 may be detected by alight receiving element. Further, the output of the fundamental wave 53may be controlled by reflecting the excitation light 27 of theexcitation laser 28 by means of an extraction mirror and detecting apart of the excitation light 27 by means of a light receiving element.By adopting these constructions, the output from the laser light source21 can be further stably obtained by more accurately detecting theoutputs of the excitation light 27 and the fundamental wave 53 to stablycontrol the output of the fundamental wave 53.

Next, specific constructions of the wavelength conversion element 25 andthe wavelength conversion element holder 35 of the wavelength converter23 are described. In this embodiment, the oscillation wavelength of thefiber laser 22 for emitting the fundamental wave 53 is changed. To thisend, it is necessary to change a phase matching condition of thenonlinear optical crystal used in the wavelength conversion element 25of the wavelength converter 23 for generating the harmonic wave 24according to the oscillation wavelength. In this embodiment is describedthe case where KTiOPO₄ (KTP) is used as an example of crystal usingangle phase matching, LiB₃O₅ (LBO) as an example of crystal usingtemperature phase matching and MgO:LiNbO₃ as an example of crystal usingquasi phase matching.

(In the Case of Using the Angle Phase Matching)

FIG. 6A shows a construction of the wavelength converter 23 in the caseof using a nonlinear crystal using the angle phase matching as thewavelength conversion element 25. In FIG. 6A, the wavelength converter23 includes a wavelength conversion element 25 a, the condenser lens 31,the beam splitter 8 and a wavelength conversion element holder 35 a. Thewavelength conversion element 25 a is formed by the angle phase matchingof the KTP crystal, and the wavelength conversion element holder 35 ahas a rotary stage for rotating the wavelength conversion element 25 ain the Φ direction on the crystal optical axis of the KTP crystal.

The oscillation wavelength of the fiber laser 22 was set at 1055 nm,1060 nm, 1065 nm and 1070 nm and the KTP crystal constituting thewavelength conversion element 25 a was used in type-II phase matching(angle θ between a z-axis and a fundamental wave incident direction is90° in an xy-plane). The respective phase matching conditions were suchthat the phase matching angle Φ (angle between the fundamental waveincident angle and an x-axis: angle in the crystal) was 35°, 32.5°,28.5° and 23.5°. It should be noted that this phase matching angle Φvaries about ±0.2° depending on the temperature of the crystal.

FIG. 6B shows another construction of the wavelength converter 23 in thecase of using the nonlinear crystal using the angle phase matching asthe wavelength conversion element 25. In FIG. 6B, the wavelengthconverter 23 includes the wavelength conversion element 25 a, thecondenser lens 31, the beam splitter 8, the wavelength conversionelement holder 35 a, a wavelength conversion element 25 b and awavelength conversion element holder 35 b. Of course, the wavelengthconversion element 25 b is formed by the angle phase matching of the KTPcrystal, and the wavelength conversion element holder 35 b has a rotarystage for rotating the wavelength conversion element 25 b in the Φdirection on the crystal optical axis of the KTP crystal. In otherwords, the construction of FIG. 6B is obtained by adding the wavelengthconversion element 25 b and the wavelength conversion element holder 35b to the construction of FIG. 6A. By this construction, a change of theoptical axis caused by the rotation of the wavelength conversion element25 a can be suppressed by the rotation of the wavelength conversionelement 25 b.

(In the Case of Using the Temperature Phase Matching)

FIG. 7 shows a construction of the wavelength converter 23 in the caseof using a nonlinear crystal using the temperature phase matching as thewavelength conversion element 25. In FIG. 7, the wavelength converter 23includes a wavelength conversion element 25 c, the condenser lens 31,the beam splitter 8 and a wavelength conversion element holder 35 c. Thewavelength conversion element 25 c is formed by the temperature phasematching of the LBO crystal, and the wavelength conversion elementholder 35 c has a heater 42 for keeping the temperature of thewavelength conversion element 25 c and a spacer 41 disposed in aclearance between the corresponding heater 42 and the wavelengthconversion element 25 c. The heaters 42 are formed by building cartridgeheaters in brass blocks, define a space that is 5 mm×5 mm×25 mm (inlongitudinal direction) and hold the wavelength conversion element 25 cmade of the LBO crystal and having dimensions of 3 mm×3 mm×20 mm in thisspace. The spacers 41 made of aluminum are disposed in the clearances.Here, the phase matching temperature indicates the temperature of athermocouple for the temperature monitor which is disposed in the brassblocks.

When the oscillation wavelength of the fiber laser 22 was set at 1055nm, 1060 nm, 1065 nm and 1070 nm and noncritical phase matchingconditions (type-I) of the LBO crystal (θ=90°, Φ=0°: on the x-axis)constituting the wavelength conversion element 25 c were used, therespective phase matching temperatures were 161° C., 155° C., 147° C.and 136° C. It should be noted that this phase matching temperaturevaries by about +2° C. depending on the individual difference of thecrystal and the holding condition in the heater 42.

(In the Case of Using the Quasi Phase Matching)

FIG. 8 shows a construction of the wavelength converter 23 in the caseof using a nonlinear crystal using the quasi phase matching as thewavelength conversion element 25. In FIG. 8, the wavelength converter 23includes a wavelength conversion element 25 d, the condenser lens 31,the beam splitter 8 and a wavelength conversion element holder 35 d. Thewavelength conversion element 25 d is formed by the quasi phase matchingof a nonlinear optical crystal (MgO: LiNbO₃[MgO:LN]), and the wavelengthconversion element holder 35 d has a Peltier element for keeping thetemperature of the wavelength conversion element 25 d, a controller 51for the Peltier element, and a moving stage 52 for moving the wavelengthconversion element 25 d in a specified direction. Since a polarizationreversal periods are formed in the nonlinear optical crystal (MgO:LN)and a maximum nonlinear optical constant d33 of the MgO:LN crystal canbe used, highly efficient wavelength conversion is possible. Since thepolarization reversal periods are specified by the wavelength of thefundamental wave 53, the polarization reversal periods formed in thenonlinear optical crystal also need to be switched every time thewavelength of the fundamental wave 53 is switched.

If the oscillation wavelength of the fiber laser 22 is set at 1055 nm,1060 nm, 1065 nm and 1070 nm, necessary polarization reversal periodsare 6.8 μm, 6.9 μm, 7 μm and 7.1 μm. FIGS. 9A to 9C shows thepolarization reversal periods formed in the MgO:LN crystal. In FIG. 9A,areas A, B, C, . . . formed with polarization reversal periods arearranged in parallel with a propagation direction of the fundamentalwave 53. For example, the polarization reversal period formed in thearea A corresponds to the fundamental wave 53 of λ1, the one formed inthe area B the fundamental wave 53 of λ2 and the one formed in the areaC the fundamental wave 53 of λ3. On the other hand, in FIG. 9B, thepolarization reversal periods linearly change. For example, thepolarization reversal period at position D corresponds to thefundamental wave 53 of λ1, the one at position E the fundamental wave 53of λ2 and the one at position F the fundamental wave 53 of λ3.

The wavelength conversion elements 25 d having polarization reversalperiod structures in FIGS. 9A and 9B need to have the area or position,on which the fundamental wave 53 is incident, switched as the wavelengthof the fundamental wave 53 is switched. Accordingly, the wavelengthconversion element 25 d is made movable by the moving stage 52 of thewavelength conversion element holder 35 d. Further, a drive timing ofthe moving stage 52 has to be synchronized with a drive timing of thereflection wavelength varying unit 33 of FIG. 1. To this end, thecontroller 34 generates a drive signal for driving the moving stage 52in synchronism with a drive signal for driving the reflection wavelengthvarying unit 33. In this way, wavelength conversion can be efficientlycarried out by accurately switching the area or position of thewavelength conversion element 25 c on which the fundamental wave 53 isincident in conformity with the reflection wavelength shift of thesecond fiber grating 30 by the reflection wavelength varying unit 33.The structure of FIG. 9A can be easily formed if the crystal axis of theMgO:LN crystal is considered.

On the other hand, in FIG. 9C, areas G, H, I, . . . formed withpolarization reversal periods are arranged one after another along thepropagation direction of the fundamental wave 53. For example, thepolarization reversal period formed in the area G corresponds to thefundamental wave 53 of λ1, the one formed in the area H the fundamentalwave 53 of λ2 and the one formed in the area I the fundamental wave 53of λ3. Since the areas of the respective polarization reversal periodsare arranged along the propagation direction of the fundamental wave 53in the structure of FIG. 9C, it is not necessary to move the wavelengthconversion element 25 d as the wavelength of the fundamental wave 53 isswitched. Accordingly, the moving stage 52 of FIG. 8 is not necessary,thereby simplifying the construction of the wavelength conversionelement holder 35 d and mitigating burdens on the process of thecontroller 34. The structure of FIG. 9C can be realized by discretechanges of the wavelength of the fundamental wave 53. However, sinceareas of wavelength conversion (mutual action length) are shorter, theconversion efficiency from the fundamental wave 53 into the harmonicwave 24 decreases. Therefore, the structure of FIG. 9A is preferable inthe case of attaching importance to the conversion efficiency.

Next, a method by which the laser light source 21 of FIG. 1 outputs ahigh-output green laser light (hereinafter, “green light”) is described.In FIG. 1, a rare-earth element Yb is doped at a concentration of 1200ppm as a laser-active substance in a core part of the fiber 26 of thefiber laser 22. A semiconductor laser having a wavelength of 915 nm, athreshold current of 400 mA and a maximum light output of 10 W is usedas the excitation laser 28 for the fiber excitation. An excitation lighthaving a wavelength of 915 nm is incident on the fiber 26 and totallyabsorbed until reaching the second fiber grating 30. As a result, if theexcitation light 27 from the excitation laser 28 is incident on thefiber 26, it is absorbed in the core part and an induced emission of alight having a wavelength of about 1050 to 1065 nm occurs from the fiber26, utilizing the Yb level of the core part. An induced emission lighthaving a wavelength of about 1050 to 1065 nm propagates in the fiber 26while being amplified with a gain obtained by the absorption of theexcitation light 27 and becomes the fundamental wave 53 that is aninfrared laser light having a wavelength of about 1050 to 1065 nm.Further, the fundamental wave 53 reciprocates between reflectionsurfaces while using the first and second fiber gratings 29, 30 as apair of reflection surfaces of the laser cavity, whereby the oscillationwavelength is selected mainly by the second fiber grating 30 having alower reflectance. As described above, the reflection wavelength of thesecond fiber grating 30 at this time is set at 1050 nm, 1055 nm, 1060 nmand 1065 nm and the reflection wavelength bandwidth thereof is set at0.1 nm. Accordingly, the fundamental wave 53 with a wavelength bandwidthof 0.1 nm is outputted from the fiber laser 22. Although thereflectances of the first and second fiber gratings 29, 30 at theoscillation wavelengths of 1050 nm, 1055 nm, 1060 nm and 1065 nm arerespectively set at about 98% and 10%, the reflectances at therespective oscillation wavelengths can be changed by designing since thefirst and second fiber gratings 29, 30 are both sampled gratings. Bysetting the reflectance of the first fiber grating 29 at 98% or higher,the oscillation light can be prevented from returning to the excitationlaser 28 to break the excitation laser 28. On the other hand, thereflectance of the second fiber grating 30 is preferably about 5 to 20%since it is sufficient to feed back only a light amount to lock adesired oscillation wavelength.

FIG. 10 shows an input/output characteristic of the light output of thefundamental wave 53 with a wavelength of 1065 nm in relation to anamount of the excitation light from the excitation laser 28. It can beunderstood that the output of the fundamental wave 53 increases withgood linearity in proportion to the amount of the excitation light up to7 W.

Next, the process of converting the fundamental wave 53 emitted from thefiber laser 22 into the harmonic wave 24 by the wavelength conversionelement 25 is described. The fundamental wave 53 (e.g. having awavelength of 1065 nm) outputted from the fiber laser 22 is incident onthe wavelength conversion element 25 via the condenser lens 31. Thewavelength conversion element 25 is an element for outputting theincident light while converting it into the harmonic wave 24, andMgO:LiNbO₃ crystal having a polarization reversal period structure witha length of 20 mm is, for example, used as such. The case where theMgO:LiNbO₃ crystal having a polarization reversal period structure isused is described below. Here, a wavelength at which the incident lightis convertible into a harmonic wave in the wavelength conversion element25 is called a phase matching wavelength, and is set at 1065 nm at 25°C. in this embodiment. Accordingly, the wavelength 1065 nm of thefundamental wave 53 of the fiber laser 22 coincides with the phasematching wavelength and the fundamental wave 53 is converted, in thewavelength conversion element 25, into the harmonic wave 24, which isoutputted from the wavelength converter 23 in the form of a green laserlight having a wavelength 532.5 nm that is half the wavelength of thefundamental wave 53.

Generally, the wavelength conversion element 25 istemperature-controlled at an accuracy of 0.01° C. since the phasematching wavelength sensitively changes depending on the temperature ofthe element. In this embodiment, the wavelength conversion element 25and the second fiber grating 30 are individually temperature-controlledat an accuracy of 0.01° C. by attaching Peltier elements. With thisarrangement, even if the output of the fundamental wave 53 of the fiberlaser 22 exceeds 5 W to increase heats generated in the wavelengthconversion element 25 and the second fiber grating 30, the harmonic wave24 in the form of a W-class green laser light can be obtained. It shouldbe noted that temperature sensors are attached to the Peltier elementsand the Peltier elements and the temperature sensors are all connectedto the controller 34 so that the capture of temperature signal outputsand the driving of the respective components and elements can becontrolled.

Accordingly, a fundamental wave having a shorter wavelength can beoutputted at a high output equal to or above 5 W by adjusting the kindor amount of a rare-earth element added to the fiber 26 or by adjustingthe reflection wavelength of the second fiber grating 30 to a shorterwavelength. Therefore, a W-class green laser light having a shorterwavelength of 526 to 540 nm can be obtained.

The controller 34 may store a table containing data inputted beforehandand temperature-control the second fiber grating 30 and the wavelengthconversion element 25 based on this table. By these constructions, thephase matching conditions of the fundamental wave 53 in the wavelengthconversion element 25 can be accurately controlled, and the more stableharmonic wave 24 can be efficiently outputted from the wavelengthconversion element 25.

The table may contain data on an amount of change of the phase matchingwavelength in the wavelength conversion element 25 in relation to theoutput of the fundamental wave 53. Alternatively, the table may containdata on an amount of change of the reflection wavelength in the secondfiber grating 30 in relation to the output of the fundamental wave 53.By these constructions, the phase matching conditions in the wavelengthconversion element 25 in relation to the output and wavelength of thefundamental wave 53 can be quickly adjusted by the temperature controlsof the second fiber grating 30 and the wavelength conversion element 25when the output of the fundamental wave 53 changes, whereby the outputof the harmonic wave from the wavelength conversion element 25 can bemore stably maintained.

A green laser light having a shorter wavelength of 526 to 540 nm can beobtained and the range of reproduced colors can be more expanded thanthe conventional sRGB specification by shortening the length of thefiber 26 of the fiber laser 22. Therefore, a color reproduction rangecan be further expanded upon the application to a display or the like.

Second Embodiment

Next, a second embodiment of the present invention is described. FIG. 11shows a construction of a laser display (two-dimensional image displaydevice) according to the second embodiment of the present invention. Thelaser display according to this embodiment is an example of a laserdisplay to which the laser light source according to the firstembodiment is applied.

Three color laser light sources 1001 a to 1001 c of red (R), green (G)and blue (B) are used as light sources. An AlGaInP/GaAs semiconductorlaser having a wavelength of 638 nm is used as the red laser lightsource (R light source) 1001 a and a GaN semiconductor laser having awavelength of 465 nm is used as the blue laser light source (B lightsource) 1001 c.

On the other hand, the laser light source according to the firstembodiment is used as the green laser light source (G light source) 1001b. This is a laser light source including the wavelength conversionelement for halving the wavelength of the infrared laser light. Laserbeams emitted from the respective R, G, B light sources 1001 a to 1001 care caused to scan diffusers 1003 a to 1003 c by reflection-typetwo-dimensional beam scanning means 1002 a to 1002 c after beingcondensed by condenser lenses 1009 a to 1009 c. An image data is dividedinto R, G and B data, and a color image is formed by multiplexingsignals of these data by means of a dichroic prism 1006 after beingfocused by field lenses 1004 a to 1004 c and inputted to spatial lightmodulation elements 1005 a to 1005 c. The image multiplexed in this wayis projected onto a screen 1008 by a projection lens 1007. A concavelens 1009 for equalizing the spot size of the green light to those ofthe R light and B light in the spatial light modulation element 1005 bis inserted in a light path from the G light source 1001 b to thespatial light modulation element 1005 b.

Although each of the R light source and B light source is constructed byone semiconductor laser in this embodiment, it may be constructed suchthat outputs of a plurality of semiconductor lasers can be obtained asone output, for example, by grouping bundle fibers together. With suchan arrangement, the widths of the wavelength spectra of the R lightsource and B light source can be increased, whereby coherency can bemitigated to suppress speckle noises of the light sources. Similarly forthe G light source, G light outputs of a plurality of semiconductorlasers may be respectively guided by output fibers, and these outputfibers may be grouped together into one fiber, for example, by usingbundle fibers so as to suppress speckle noise.

In the laser display of FIG. 11, members such as vibration diffusers1003 a to 1003 c and the field lenses 1004 a to 1004 c are disposedbefore the spatial light modulation elements 1005 a to 1005 c. Thesemembers are disposed to remove speckle noises generated by the use oflaser beams having strong coherency as light sources. By swinging thesespeckle noise removing means, speckle noise seen during a response timeof human eyes can be reduced.

In this embodiment, a fundamental wave emitted from a fiber laser isincident on a wavelength conversion element to generate a harmonic waveby using the laser light source of the first embodiment as the G lightsource 1001 b. The construction of the laser display according to thisembodiment is characterized by the laser light source used as the Glight source 1001 b.

The laser display of this embodiment can have a high luminance and athin configuration since the laser light sources are used as the R, Gand B light sources. Further, the color reproduction range can be moreexpanded, for example, to 523 nm than the conventional sRGBspecification and color representation approximate to original colors ispossible by using the laser light source according to the firstembodiment as the G light source 1001 b. In other words, the laserdisplay of this embodiment can expand the color reproduction range morethan conventional laser displays.

Further, the G light source 1001 b can arbitrarily oscillate any oflights, for example, having wavelengths of 525 nm, 527.5 nm, 530 nm and532.5 nm. Thus, speckle noise can be reduced to 20% or lower as comparedto the case of a single wavelength. Further, by swinging the vibrationdiffuser 1003 b, a brightness difference caused by the speckle noise canbe reduced to such an extent (2% or lower) that the speckle noise cannotbe sensed by human eyes.

This embodiment can have such a mode for projecting a light from behindthe screen (rear projection display) besides the two-dimensional imagedisplay device having such a construction. Further, this embodiment canalso be used as a backlight of a liquid crystal panel by evening out thelaser lights by means of light guide plates.

Third Embodiment

Next, a third embodiment of the present invention is described.Generally, it is known that the color reproduction range of a videochanges as the oscillation wavelength of a green light changes. FIG. 12shows a relationship between the wavelength of a green light and thecolor reproduction range. Since a luminosity factor is high at awavelength of 532.5 nm, a small power is sufficient to obtain the sameluminance, but there is a problem of being unable to reproduce ‘cyan’colors necessary to display the color of the sea and the like. On theother hand, the “cyan” colors can be reproduced at a wavelength of 525nm, but there is a problem of necessitating more than twice the power ascompared to the case of 532.5 nm since a luminosity factor is low.

In order to solve such problems, a laser light source according to thisembodiment enables more bright videos to be displayed with the samepower consumption utilizing the luminosity factor of human eyes byswitching the oscillation wavelength of a laser light depending on thetype of the video and a status of use. A case where the laser lightsource according to this embodiment is used as a green light source of alaser display is described below.

FIG. 13 shows a schematic construction of the laser light sourceaccording to this embodiment. A laser light source 110 according to thisembodiment is constructed by adding a projector control circuit 1101including a wavelength determining circuit 1102 and a luminance signaljudging circuit 1103, and a video mode changeover switch 1104 to thelaser light source according to the first embodiment. Further, a videosignal (data) 1105 or a video signal (video) 1106 is externally inputtedto the projector control circuit 1101, which in turn sends a wavelengthselection signal 1107 to a controller 34 to select the oscillationwavelength of the laser light.

The operation of the projector control circuit 1101 is described.Normally, a laser display includes a plurality of terminals for theinput and output of video signals such as D-sub15pin, DVI, RCA pin,S-terminal, D-terminal and HDMI. Accordingly, there is first describedthe case where the projector control circuit 1101 changes theoscillation wavelength of the laser light by detecting to which one ofthe terminals of the laser display a video signal has been inputted.

It is, for example, assumed that the video signal (data) 1105 has beeninputted to the laser display. If the video signal (data) 1105 has beeninputted through the D-sub15pin or DVI, this video signal 1105 can besaid to be a data signal in which importance is attached to brightnesssuch as the one used for presentation. In this case, the wavelengthdetermining circuit 1102 sends the wavelength selection signal 1107 tothe controller 34 to select a green light having a wavelength with ahigh luminosity factor.

If the video signal has been inputted through the RCA pin, S-terminal,D-terminal, HDMI or the like, this video signal is often the videosignal (video) 1106. In this case, the luminance signal judging circuit1103 judges the brightness of a video source. The luminance signaljudging circuit 1103 analyzes a luminance signal in the video todiscriminate whether the inputted video signal 1106 is a video signal inwhich much importance is not attached to color because of a bright scenesuch as general television programs (e.g. programs shot in studios) or avideo signal requiring a wide color reproduction range although thereare many dark scenes as in a movie. In the former case, the rate ofusing green wavelengths with a high luminosity factor can be increasedto improve efficiency. In the latter case, the rate of increasingshorter green wavelengths such as 526 nm capable of expanding the colorreproduction range can be increased to improve image quality.

Further, a user can arbitrarily determine which wavelength to be used bymeans of the video mode changeover switch 1104. For example, a greenwavelength having a high luminosity factor can be designated if the userprefers bright videos, whereas a wavelength capable of expanding thecolor reproduction range can be designated if the user constantly wantsto see high quality videos having a wide color reproduction range. Theoscillation wavelength can also be determined in the wavelengthselection signal 1107 determined by the luminance signal judging circuit1103.

Since the laser light source 110 according to this embodiment canoscillate at any desired one of the wavelengths 525 nm, 527.5 nm, 530 nmand 532.5 nm, brightness sensed by human eyes can be improved with thesame power consumption by emitting a light having a wavelength of 532.5nm with a high luminosity factor if the laser light source 110 is usedin a data projector requiring more brightness than colorreproducibility. On the other hand, in the case of requiring more colorreproducibility than brightness as in a movie, color reproducibility canbe improved by emitting a light having a wavelength of 525 nm capable ofexpanding the color reproduction range although having a low luminosityfactor.

The method for changing an emission ratio is not limited to theaforementioned one, and similar effects can be obtained even if anothermethod is applied.

In this embodiment, in the case of using a battery 1109 as a powersource, the life of the battery 1109 can be improved by using either oneof an AC power source 1108 or the battery 1109 or by switching theoscillation wavelength depending on the remaining amount of the battery1109. For example, in the case of judging which of the AC power source1108 and the battery 1109 is being used, a power source control circuit1110 judges the type of the power source and sends a power sourcejudgment signal 1111 to the wavelength determining circuit 1102, whichin turn determines the wavelength. By determining the oscillationwavelength through the judgment on the remaining amount of the battery1109 by the power source control circuit 1110, a bright image can bedisplayed with less power consumption by projecting a green light havinga higher efficiency as the fiber laser 22 and a longer wavelength with ahigh luminosity factor if the battery 1109 is used or the remainingbattery amount is small.

Fourth Embodiment

Next, a fourth embodiment of the present invention is described. FIG. 14shows a construction of a laser fluorescence microscope according to thefourth embodiment of the present invention. The laser fluorescencemicroscope according to this embodiment uses the laser light sourceaccording to the first embodiment, and observes a fluorescent part bydying a sample with a fluorescent substance such as rhodamine andexciting the dyed sample with a light emitted from the laser lightsource according to the first embodiment. There are several kinds offluorescent substances, and the wavelengths of corresponding excitationlights differ. For example, the excitation wavelength of a fluorescentreagent called TRITC is 540 nm, and that of a reagent called TexasRed is560 nm. Besides, fluorescent reagents having various excitationwavelengths are commercially available. Since parts remaining in cellsdiffer among various reagents, visibility can be largely improved.

A laser light emitted from a laser light source 21 of a laserfluorescence microscope 1301 according to this embodiment is reflectedby mirrors 1302 a, 1302 b to be introduced into a microscope 1303. Byirradiating a sample 1304 with the laser light, a fluorescent substanceheld in the sample 1304 is excited, thereby enabling a fluorescent partto be confirmed in the form of an image. Conventionally, a light sourcethat splits a light of a halogen lamp by means of a dichroic mirror anda light source using a dye laser have been used, but the light sourceusing the halogen lamp has had a problem of exciting dyes desired not beexcited due to incapability of complete color separation. The lightsource using the dye laser has had a problem of requiring dyes of aliquid to be frequently exchanged and requiring a large number ofmaintenances although the complete color separation is possible becauseof being a tunable laser. This embodiment is free from maintenance andhas a remarkably smaller size than the dye laser although the wavelengthis discretely changed.

As described above, according to the first to fourth embodiments, theoscillation wavelength of the laser can be discretely changed by makingthe fiber grating periods at a narrower band side of a pair of fibergratings having a plurality of reflection wavelengths variable. Thus,there can be realized a high-output laser light source device that canbe used while switching a plurality of wavelengths. Further, since thewavelength at which light is desired to be oscillated can be arbitrarilyselected by conforming the reflectance at the narrower band side and thegain of the laser medium to each other, a W-class visible laser lightcan be more stably outputted and a green laser light having a higherluminosity factor can be emitted as compared to the case of simultaneousoscillation at a plurality of wavelengths. There can be also realized atwo-dimensional image display device having a high luminance, a widecolor reproduction range, a high image quality and a low powerconsumption.

In the above first to fourth embodiments, amplified spontaneous emissionoccurs in a wavelength range of 1000 to 1100 nm in the fiber laser tobreak the excitation laser in the case of obtaining a harmonic wave byoscillating a light having a wavelength equal to or longer than 1100 nmsuch as 1120 nm as shown in FIG. 15A. Accordingly, the breakage of theexcitation laser can be prevented and a stable output can be obtained bydesigning the fiber gratings so as to simultaneously oscillate at anywavelengths in the wavelength range of 1000 to 1100 nm as shown in FIG.15B.

Further, although the fiber laser doped with Yb as a rare-earth elementis used in the above first to fourth embodiments, at least onerare-earth element selected from other rare-earth elements such as Ndand Er may be used. It is also possible to change a doped amount of therare-earth element and to dope a plurality of rare-earth elementsdepending on the wavelength and output of the wavelength conversionelement.

Although the laser having wavelengths of 915 nm and 976 nm is used asthe excitation laser of the fiber laser in the above first to fourthembodiments, any laser having other wavelengths may be used providedthat it can excite the fiber laser.

Since the laser light source and the two-dimensional image displaydevice according to the first to fourth embodiments have a highluminance, a wide color reproduction range and a low power consumption,they are useful for analytical application in the display field such aslarge-scale displays and high-luminance displays and in the biochemicalfield.

Fifth Embodiment

Next, a fifth embodiment of the present invention is described. Thisembodiment relates to a medical laser light source device used forsurgery, which device uses a laser light source capable of outputting astable visible high-output laser light.

FIG. 16 is a diagram showing a schematic construction of a laser lightsource device according to this embodiment. A laser light source device2100 according to this embodiment is provided with a laser oscillator2101 and a laser light irradiator 2102. The laser oscillator 2101includes a fiber laser 2103, a power source 2104, a controller 2105,whereas the laser light irradiator 2102 includes two wavelengthconversion elements 2111 and 2112 and a light selector 2113. The laseroscillator 2101 also includes a turn-on switch 2106 with which a usercan determine an emission timing of a laser light. The laser lightemitted from the fiber laser 2103 is transmitted to the laser lightirradiator 2102 via fibers 2117.

The fiber laser 2103 includes a fiber 2110 doped with a rare earth suchas Yb, an excitation laser 2107 for outputting an excitation light to beincident on the fiber 2110, fiber gratings 2108 a, 2108 b, 2109 a and2109 b capable of selecting the wavelength of a fundamental wave, and aPANDA coupler 2110 a, wherein the fiber gratings 2108 a and 2109 a andthe fiber 2110 are formed by double-clad polarization maintainingfibers.

The laser light irradiator 2102 feeds parts of the generated visiblelight back to the laser oscillator 2101 via the fibers 2117. In thelaser oscillator 2101, the visible lights fed back via the fibers 2117are monitored by photodiodes 2115, 2116. A controller 2105 stabilizes anoutput of the excitation laser 2107 by controlling the power source 2104for driving the excitation laser 2107 based on a monitoring result. Thefibers 2117 are preferably double-clad polarization maintaining fibersin order to reduce a coupling loss of the visible light from the laserlight irradiator 2102 to the fibers 2117 and a propagation loss of thevisible light by the fibers 2117. Further, leak lights are generated atconnection points of the fiber gratings 2108 b, 2109 b with the fibers2117 by forming the fiber gratings 2108 b, 2109 b of ordinarypolarization maintaining fibers. Thus, visible lights for monitoring canbe obtained for the photodiodes 2115, 2116.

Next, the operation of the fiber laser 2103 is described. An excitationlight from the excitation laser 2107 is incident on the fiber 2110. Theincident excitation light propagates in the fiber 2110 while beingabsorbed by a laser-active substance contained in the fiber 2110. A seedlight of the fundamental wave is generated in the fiber 2110 in a statewhere gains for amplifying the fundamental wave in the fiber 2110 areuniformly increased through the passage and absorption of the excitationlight in the fiber 2110. This seed light of the fundamental wavereciprocates by being reflected in these cavities while increasing theintensity by being amplified and consequently reaches laser oscillation,using the fiber gratings 2108 a, 2108 b as one cavity or using the fibergratings 2109 a, 2109 b as one cavity.

The PANDA coupler 2110 a is used to change the oscillation wavelength ineach polarization direction of the fundamental wave. For example, byfusion bonding the fiber grating 2109 b having a reflection wavelengthof 1063 nm and a reflection band of 0.05 nm in a slow-axis direction ofthe PANDA coupler 2110 a and fusion bonding the fiber grating 2108 bhaving a reflection wavelength of 1178 nm and a reflection band of 0.05nm in a fast-axis direction of the PANDA coupler 2110 a, oscillationoccurs at 1064 nm in the slow-axis direction and at 1178 nm in thefast-axis direction in the Yb fiber 2110. Similar effects can beobtained even if the fast and slow-axis directions of the respectivewavelengths are switched provided that polarization directions at therespective wavelengths are orthogonal to each other.

By changing the oscillation wavelength in each polarization direction ofthe fundamental wave in this way, an output instability caused byintermode competition occurring at the time of multiwavelengthoscillation can be prevented. In the case of generating a light of 1178nm, an ASE (amplitude spontaneous emission) light having a wavelength of1040 to 1090 nm is generated. This results in not only a reducedefficiency in light generation at 1178 nm, but also an inadvertentoccurrence of pulse oscillation to break the fiber laser cavity. Thus,it is caused to simultaneously oscillate at 1064 nm at the time ofgenerating the light having a wavelength of 1178 nm. By doing so,inadvertent pulse oscillation is suppressed to prevent the breakage ofthe fiber laser cavity. At this time, if a desired wavelength is 589 nmthat is the wavelength of a harmonic wave of the fundamental wave havinga wavelength of 1178 nm, a Q-value of the cavity generating the lighthaving a wavelength of 1064 nm is decreased by changing the reflectionwavelength of the fiber grating 2109 b through the control of thetemperature of the fiber grating 2109 b or a tensile stress to the fibergrating 2109 b for generating the light having a wavelength of 1064 nm.In this way, the energy of the excitation light is more used for thegeneration of the light having a wavelength of 1178 nm by the fibergrating 2108 b.

As described above, it is essential that the fiber gratings 2108 a, 2109a and the fiber 2110 are double-clad polarization maintaining fibers. Onthe other hand, the PANDA coupler 2110 a, the fiber gratings 2108 b and2109 b are preferably ordinary polarization maintaining fibers having nodouble-clad structure. The tail ends of the polarization maintainingfibers formed with the fiber gratings 2108 b and 2109 b, i.e. exits forthe fundamental wave are connected by fusion bonding with the fibers2117 for transmitting the fundamental wave laser light to the laserlight irradiator 2102. The photodiodes 2115, 2116 for monitoring returnlights of visible lights generated in the wavelength conversion elements2111, 2112 arranged in the laser light irradiator 2102 are deposed nearthe connection points. Since infrared lights also leak at the connectionpoints, infrared filters are preferably mounted on the photodiodes 2115,2116. In this embodiment, dielectric multilayer films for returning only8% of the visible light generated by the wavelength conversion in anincident direction are formed on the light emission surfaces of thewavelength conversion elements 2111, 2112. The visible lights reflectedin the incident direction are coupled to the fibers 2117 again. At thistime, since the fibers 2117 are double-clad fibers, the couplingefficiency of the visible lights to the fibers 2117 is improved and morevisible lights can be transmitted to the above connection points. As aresult, the photodiodes 2115, 2116 can more accurately monitor the abovevisible lights. Reflected amounts of the visible lights by therespective light emission surfaces of the wavelength conversion elements2111, 2112 depend on the lengths of the fibers 2117 and preferably liein a range of 1 to 10%.

The laser light source device 2100 according to this embodiment controlsthe phase matching conditions of the wavelength conversion elements2111, 2112 by changing the oscillation wavelength of the fiber laser2103. This control of the phase matching conditions is described below.

In FIG. 16, the visible lights reflected by the light emission surfacesof the wavelength conversion elements 2111, 2112 of the laser lightirradiator 2102 are transmitted to the laser oscillator 2101 via thefibers 2117 to be monitored by the photodiodes 2115, 2116 as describedabove. The controller 2105 obtains the output intensities of therespective visible lights from the wavelength conversion elements 2111,2112 based on the monitoring results of the photodiodes 2115, 2116. Thecontroller 2105 changes the oscillation wavelength of the fiber laser2103 by shifting the reflection wavelengths of the fiber gratings 2108b, 2109 b based on the output intensities of the respective visiblelights from the wavelength conversion elements 2111, 2112. The phasematching conditions of the wavelength conversion elements 2111, 2112 arecontrolled by changing the oscillation wavelength of the fiber laser2103.

The setting of the shift amounts of the reflection wavelengths of thefiber gratings 2108 b, 2109 b is realized, for example, through atemperature control by Peltier elements. The controller 2105 changes theoscillation wavelength of the fiber laser 2103 through the temperaturecontrol of the fiber gratings 2108 b, 2109 b to bring the phase matchingconditions of the wavelength conversion elements 2111, 2112 intocoincidence again if the output intensities vary due to the variance ofthe phase matching conditions of the wavelength conversion elements2111, 2112. It should be noted that the shift amounts of the reflectionwavelengths of the fiber gratings 2108 b, 2109 b may also be controlledby applying tensile stresses to the fiber gratings 2108 b, 2109 b as inthe first embodiment.

Next, the temperature control of the fiber gratings 2108 b, 2109 b bythe controller 2105 is specifically described. Since the sametemperature control is carried out for both the fiber gratings 2108 band 2109 b, the case of temperature controlling the fiber grating 2108 bis described below. FIG. 17A is a graph showing a change of a phasematching wavelength when the temperature of the wavelength conversionelement 2112 decreased, and FIG. 17B is a graph showing a change of thephase matching wavelength when the temperature of the wavelengthconversion element 2112 increased. The fiber grating 2108 b istemperature controlled by the Peltier element so as to reach apredetermined standby temperature. The standby temperature may be, forexample, a temperature at which the output intensity of the wavelengthconversion element 2112 is 85 to 95% of the peak output intensitythereof and which is lower than the phase matching temperature.

First, in FIG. 17A, if the temperature of the wavelength conversionelement 2112 decreases, the output of the wavelength conversion element2112 increases as shown by an arrow A1 in FIG. 17A. At this time, acharacteristic curve of the phase matching wavelength is shifted towarda shorter wavelength side as shown by an arrow A2 in FIG. 17A.Accordingly, the controller 2105 causes the oscillation wavelength ofthe fundamental wave emitted from the fiber laser 2103 to shift towardthe shorter wavelength side by decreasing the temperature of the fibergrating 2108 b. Thus, the output of the wavelength conversion element2112 decreases as shown by an arrow A3 in FIG. 17A to return to theoutput at the standby temperature of the wavelength conversion element2112.

On the other hand, if the temperature of the wavelength conversionelement 2112 increases, the output of the wavelength conversion element2112 decreases as shown by an arrow B1 in FIG. 17B. At this time, thecharacteristic curve of the phase matching wavelength is shifted towarda longer wavelength side as shown by an arrow B2 in FIG. 17B.Accordingly, the controller 2105 causes the oscillation wavelength ofthe fundamental wave emitted from the fiber laser 2103 to shift towardthe longer wavelength side by increasing the temperature of the fibergrating 2108 b. Thus, the output of the wavelength conversion element2112 increases as shown by an arrow B3 in FIG. 17B to return to theoutput at the standby temperature of the wavelength conversion element2112.

FIG. 18 is a flow chart showing the procedure of the aforementionedprocess of temperature controlling the fiber grating 2108 b by thecontroller 2105. First, the controller 2105 obtains the monitoringresult on the output intensity of the return light from the wavelengthconversion element 2112 by the photodiode 2116 (Step S101), and judgeswhether the output of the wavelength conversion element 2112 isincreasing or decreasing (Step S102).

Subsequently, the controller 2105 decreases an average current flowinginto the Peltier element of the fiber grating 2108 b (Step S103) if itis judged that the output of the wavelength conversion element 2112 isincreasing in Step S102. In this way, the temperature of the Peltierelement is decreased to decrease the temperature of the fiber grating2108 b, whereby the oscillation wavelength of the fundamental waveemitted from the fiber laser 2103 is shifted toward the shorterwavelength side (see FIG. 17A).

Subsequently, the controller 2105 confirms that the value of drivecurrent for the excitation laser 2107 supplied by the power source 2104lies within a specified set range and also confirms the output intensityof the wavelength conversion element 2112 (Step S104). Then, thecontroller 2105 compares the drive current value from the power source2104 and an initial current value, and ends the process if a differencebetween the two values lies within a specified set range (YES in StepS105) while repeating the above Steps S101 to S105 again if thisdifference lies outside the specified set range (NO in Step S105).

On the other hand, if it is judged that the output of the wavelengthconversion element 2112 is decreasing in Step S102, the controller 2105increases the average current flowing into the Peltier element of thefiber grating 2108 b (Step S106). In this way, the temperature of thePeltier element is increased to increase the temperature of the fibergrating 2108 b, whereby the oscillation wavelength of the fundamentalwave emitted from the fiber laser 2103 is shifted toward the longerwavelength side (see FIG. 17B). Then, this process proceeds to StepS105.

In this way, the aforementioned temperature control of the fiber grating2108 b by the controller 2105 is carried out.

In this embodiment, the bandwidths of the reflection wavelengths of thefiber gratings 2108 b, 2109 b are preferably 1 nm or longer in order tocorrespond to the shift amounts of the reflection wavelengths of thefiber gratings 2108 b, 2109 b. In order to suppress a change in theoscillation efficiency of the cavity caused by the shifts of thereflection wavelengths of the fiber gratings 2108 b, 2109 b, the topshapes of the bands of the reflection wavelengths of the fiber gratings2108 a, 2109 a are preferably minimally rippled, i.e. as flat aspossible.

Although the temperatures of the fiber gratings 2108 b, 2109 b arecontrolled in this embodiment, the fiber gratings 2108 a, 2108 b and thefiber gratings 2109 a, 2109 b may be paired and temperature controlledtogether. Further, each pair may be sealed in a temperature compensatingpackage. By doing so, effects similar to the above can be obtained evenif the bands of the reflection wavelengths of the fiber gratings 2108 aand 2109 a are 0.2 to 1 nm.

By controlling the output intensities of the visible lights wavelengthconverted based on the oscillation wavelength of the fiber laser 2103 inthis way, wiring from the laser oscillator 2101 to the laser lightirradiator 2102 can be reduced, thereby improving a degree of freedom indesigning the laser light irradiator 2102. Further, since the wiring andthe like that stands as a hindrance upon performing a surgery whileactually holding the laser light irradiator 2102 by the hand arereduced, the usability of the laser light irradiator 2102 can beimproved.

Next, a wavelength change of the emitted light by the operation of thelaser light irradiator 2102 is described. FIGS. 19A to 19C show aschematic construction of the laser light irradiator 2102.

As shown in FIGS. 19A to 19C, the laser light irradiator 2102 includesthe two wavelength conversion elements 2111, 2112 and the light selector2113, wherein the light selector 2113 is comprised of a pedestal 2401movable in directions shown by arrows in FIGS. 19A to 19C, a firstdielectric multilayer film mirror 2402 arranged on the pedestal 2401 forreflecting only infrared lights, and a second dielectric multilayer filmmirror 2403 arranged adjacent to the first dielectric multilayer filmmirror 2402 on the pedestal 2401 for reflecting only visible lights.

FIG. 19A shows a state where the second dielectric multilayer filmmirror 2403 is located to face the light emission surface of thewavelength conversion element 2111 for emitting a green light 2111 a. Inthis case, the green light 2111 a is caused to be emitted from the laserlight irradiator 2102 since the second dielectric multilayer film mirror2403 for reflecting only visible lights is located before the lightemission surface of the wavelength conversion element 2111.

FIG. 19B shows a state where the second dielectric multilayer filmmirror 2403 is located to face the light emission surface of thewavelength conversion element 2112 for emitting an orange light 2112 a.In this case, the orange light 2112 a is caused to be emitted from thelaser light irradiator 2102 since the second dielectric multilayer filmmirror 2403 for reflecting only visible lights is located before thelight emission surface of the wavelength conversion element 2112.

FIG. 19C shows a state where the first dielectric multilayer film mirror2402 is located to face the light emission surface of the wavelengthconversion element 2111 for emitting an infrared light 2111 b. In thiscase, the unconverted infrared light 2111 b out of the fundamental waveincident on the wavelength conversion element 2111 is caused to beemitted from the laser light irradiator 2102 since the first dielectricmultilayer film mirror 2402 for reflecting only infrared lights islocated before the light emission surface of the wavelength conversionelement 2111.

In this way, the wavelength of the emitted light can be changed by theoperation of the laser light irradiator 2102.

In this embodiment, the oscillation efficiency of the fiber laser 2103can be optimized by controlling the fiber laser 2103 of the laseroscillator 2101 in accordance with the wavelength of the light emittedfrom the laser light irradiator 2102. This optimization of theoscillation efficiency is described below.

For example, the light emitted from the laser light irradiator 2102 isthe green light 2111 a in the state of FIG. 19A. Accordingly, thefundamental wave to be emitted from the laser oscillator 2101 is a lighthaving a wavelength of 1064 nm that becomes a fundamental wave of thegreen light 2111 a emitted from the wavelength conversion element 2111,and a light having a wavelength of 1178 nm that becomes a fundamentalwave of the orange light emitted from the wavelength conversion element2112 is unnecessary. Accordingly, in this case, the controller 2105performs the temperature control of the fiber grating 2108 b or thetensile stress application control so that the reflection wavelengthband of the fiber grating 2108 b for oscillating the light having awavelength of 1178 nm departs from the reflection wavelength band of thefiber grating 2108 a. By doing so, the oscillation of the light having awavelength of 1178 nm can be stopped to utilize all the excitationenergy of the excitation laser 2107 for the oscillation of the lighthaving a wavelength of 1064 nm by the fiber grating 2109 b. Therefore,the oscillation efficiency of the light having a wavelength of 1064 nmby the fiber grating 2109 b can be improved.

FIG. 20A shows a relationship between the oscillation wavelength of thefiber laser 2103 and the reflection wavelength bands of the fibergratings 2108 a, 2108 b, 2109 a and 2109 b. As described above, thereflection wavelength band of the fiber grating 2108 b departs from thatof the fiber grating 2108 a at an oscillation wavelength of 1178 nm, andthe reflection wavelength band of the fiber grating 2109 b is locatedwithin the reflection wavelength band of the fiber grating 2109 a at anoscillation wavelength of 1064 nm.

Next, in the state of FIG. 19B, the irradiated light from the laserlight irradiator 2102 is the orange light 2112 a. Accordingly, thefundamental wave to be emitted from the laser oscillator 2101 is a lighthaving a wavelength of 1178 nm that becomes a fundamental wave of theorange light 2112 a emitted from the wavelength conversion element 2112,and a light having a wavelength of 1064 nm that becomes a fundamentalwave of the green light emitted from the wavelength conversion element2111 is unnecessary. However, if the oscillation of the light having awavelength of 1064 nm is completely stopped as in the case of generatingthe green light, the fiber laser 2103 is broken by the generation ofgiant pulses of the ASE light. Thus, the controller 2105 performs thetemperature control of the fiber grating 2109 b or the tensile stressapplication control such that the reflection wavelength band of thefiber grating 2109 b for oscillating the light having a wavelength of1064 nm overlaps the edge of the reflection wavelength band of the fibergrating 2109 a. By doing so, the light having a wavelength of 1064 nmcan be caused to weakly oscillate and the giant pulses of the ASE lightcan be prevented. Thus, most of the excitation energy of the excitationlaser 2107 can be utilized for the oscillation of the light having awavelength of 1178 nm by the fiber grating 2108 b. Therefore, theoscillation efficiency of the light having a wavelength of 1178 nm bythe fiber grating 2108 b can be improved without generating the ASElight.

FIG. 20B shows a relationship between the oscillation wavelength of thefiber laser 2103 and the reflection wavelength bands of the fibergratings 2108 a, 2108 b, 2109 a and 2109 b. As descried above, thereflection wavelength band of the fiber grating 2109 b overlaps the edgeof the reflection wavelength band of the fiber grating 2109 a at theoscillation wavelength of 1064 nm, and the reflection wavelength band ofthe fiber grating 2108 b is located within the reflection wavelengthband of the fiber grating 2108 a.

Next, in the state of FIG. 19C, the emitted light from the laser lightirradiator 2102 is the infrared light 2111 b that is the fundamentalwave to be incident on the wavelength conversion element 2111.Accordingly, the fundamental wave to be emitted from the laseroscillator 2101 is a light having a wavelength of 1064 nm that becomes afundamental wave of the green light 2111 a emitted from the wavelengthconversion element 2111, and a light having a wavelength of 1178 nm thatbecomes a fundamental wave of the orange light emitted from thewavelength conversion element 2112 is unnecessary. Accordingly, in thiscase, the controller 2105 performs the temperature control of the fibergrating 2108 b or the tensile stress application control such that thereflection wavelength band of the fiber grating 2108 b for oscillatingthe light having a wavelength of 1178 nm departs from the reflectionwavelength band of the fiber grating 2108 a. By doing so, theoscillation of the light having a wavelength of 1178 nm can be stoppedto utilize all the excitation energy of the excitation laser 2107 forthe oscillation of the light having a wavelength of 1064 nm by the fibergrating 2109 b. Therefore, the oscillation efficiency of the lighthaving a wavelength of 1064 nm by the fiber grating 2109 b can beimproved. By not meeting the phase matching condition of the wavelengthconversion element 2111, the wavelength conversion efficiency by thewavelength conversion element 2111 is reduced, whereby an extractedamount of the unconverted infrared light 2111 b can be increased.

Further in this embodiment, a laser light emission time can bedetermined by a user by using the turn-on switch 2106 of the laseroscillator 2101 and a shutter 2114 of the laser light irradiator 2102,whereby the power consumption of the fiber laser 2103 can be reduced.This reduction of the power consumption is described below.

FIG. 21 is a flow chart showing the procedure of an irradiation processof the laser light irradiator 2102 by the turn-on switch 2106 and thelaser light irradiator 2102. First, when the turn-on switch 2106 isturned on by a user of the laser light source device 2100 (Step S201), aturn-on instruction is notified from the turn-on switch 2106 to thecontroller 2105 (Step S202).

The controller 2105 controls the power source 2104 in accordance withthe turn-on instruction from the turn-on switch 2106, thereby causingthe excitation laser 2107 to be turned on by a predetermined drivecurrent supplied to the excitation laser 2107 (Step S203). Thecontroller 2105 monitors, by means of the photodiodes 2115, 2116, returnlights of visible lights emitted from the wavelength conversion elements2111, 21112 when the excitation laser 2107 is turned on. Using thesemonitoring results, the controller 2105 controls the reflectionwavelengths of the fiber gratings 2108 b, 2109 b so as to maximize theoutput intensities of the wavelength conversion elements 2111, 2112,thereby changing the oscillation wavelength of the fiber laser 2103(Step S204). Here, the control of the reflection wavelengths of thefiber gratings 2108 b, 2109 b may be performed through the temperaturecontrol or the tensile stress application control as described above.

Subsequently, the controller 2105 causes the shutter 2114 of the laserlight irradiator 2102 to be opened and causes emitted lights of thewavelength conversion elements 2111, 2112 to be irradiated from thelaser light irradiator 2102 (Step S206) when the outputs of thewavelength conversion elements 2111, 2112 are stabilized (YES in StepS205). On the other hand, Steps S204 and S205 are repeated unless theoutputs are stabilized in Step S205 (NO in Step S205).

In this way, the light irradiating operation by the turn-on switch 216and the laser light irradiator 2102 is performed.

In this embodiment, by adopting a method for changing the oscillationwavelength of the fiber laser 2103 as a method for satisfying the phasematching conditions of the wavelength conversion elements 2111, 2112,the light emitting operation from the operation of the turn-on switch2106 to the emission of a visible laser light can be performed within aperiod of 200 μs or shorter.

In the case of satisfying the phase matching conditions through thetemperature controls of the wavelength conversion elements 2111, 2112,the wavelength conversion elements 2111, 2112 preferably have small heatcapacities. This is because time constants at the time of thetemperature controls can be made smaller and the phase matchingconditions can be satisfied within a shorter period of time by using thewavelength conversion elements having small heat capacities.

By shortening the period from the operation of the turn-on switch 2106to the light irradiation from the laser light irradiator 2102 in thisway, it is no longer necessary to keep the fiber laser 2103 on standbyand remarkably lower power consumption can be realized.

In this embodiment, in the case of using a different laser lightirradiator 2102 for each oscillation wavelength, the irradiation rangeof the laser light irradiator 2102 can be determined by attaching aprobe to the leading end of the laser light irradiator 2102. FIG. 22shows a schematic construction in which a probe is attached to the lightemission surface of the laser light irradiator 2102.

As shown in FIG. 22, a probe 2701 is provided on the light emissionsurface of the laser light irradiator 2102. A reflection mirror 2702having a reflectance of, e.g. about 10% relative to the light emittedfrom the wavelength conversion element 2111. The light emitted from thewavelength conversion element 2111 is partly reflected by the reflectionmirror 2702, and this reflected light passes through the wavelengthconversion element 2111 again and the fiber 2117 to be monitored by thephotodiode 2115 of the laser oscillator 2101. In this way, whether ornot the probe is attached can be detected based on the presence orabsence of the above reflected light, whereby the erroneous use of thelaser light irradiator 2102 can be prevented. If the type of the probeused differs depending on the wavelength range of the emitted light, theerroneous use of the probe for the wavelength in use can be prevented bydisposing a reflection mirror suitable for the reflection of lights inthe wavelength range suited to each probe.

In this embodiment, a stable visible laser light having a desired outputcan be obtained from the startup of the laser by correcting theexcitation energy of the excitation laser 2107. For example, there maybe provided a correction mode for correcting the monitoring results bythe photodiodes 2115, 2116 of the laser oscillator 2101 and actualoutput intensities of the wavelength conversion elements 2111, 2112. Acorrelation between actual output intensity and a monitoring result isobtained from the actual output intensities and the monitoring resultsof the photodiodes 2115, 2116 by directly taking the light emitted fromthe laser light irradiator 2102 into the laser oscillator 2101. Thiscorrelation data is stored as a correction data in a storage medium suchas a register in the controller 2105. By this operation, a stablevisible laser light having a desired output can be obtained from thestartup of the excitation laser 2107.

It has been known that the fiber gratings 2108 b, 2109 b are heated byoscillation lights upon an increase of the output of the fiber laser2103, thereby causing nonuniform thermal expansion to widen the width ofreflection wavelength band as shown in FIG. 23. For example, FIG. 24shows a relationship between a wavelength bandwidth of the fundamentalwave of the laser light of the fiber laser 2103 and wavelengthconversion efficiency from the fundamental wave into the harmonic wave(standardized wavelength conversion efficiency) in the case of formingthe wavelength conversion elements 2111, 2112 by the quasi phasematching of Mg:LiNbO₃ crystal having a polarization reversal periodlength of 25 mm. As shown in FIG. 24, as a result of widening the widthsof the reflection wavelength bands of the fiber gratings 2108 b, 2109 b,there are problems of widening the wavelength bandwidth of thefundamental wave emitted from the fiber laser 2103 and reducing thewavelength conversion efficiency by the wavelength conversion elements2111, 2112. Normally, the maximum wavelength conversion efficiency ofthe wavelength conversion element using Mg:LiNbO₃ crystal is about6.5%/W, but it becomes difficult to maximally increase the wavelengthconversion efficiency if the wavelength bandwidth of the fundamentalwave becomes larger. Thus, the wavelength bandwidth of the fundamentalwave of the laser light of the fiber laser 2103 is preferably 0.6 nm orshorter in order to use the wavelength conversion element at 90% orhigher of the maximum wavelength conversion efficiency. In thisembodiment, partial thermal expansion can be evened out by fixing thefiber gratings 2108 b, 2109 b while applying tensile stresses to them inview of this problem. Therefore, as shown in FIG. 25, the spread of thebandwidth of the fundamental wave is suppressed and a reduction in thewavelength conversion efficiency from the fundamental wave into thevisible light can be prevented even if the output of the fiber laser2103 increases.

Although a laser having wavelengths of 915 nm and 975 nm is generallyused as the excitation laser 2107 of the fiber laser 2103 in thisembodiment, a laser light source having wavelengths other than these maybe used if it can excite the fiber laser 2103.

Further, although MgO:LiNbO₃ having a polarization reversal periodstructure is used for the wavelength conversion elements 2111, 2112, awavelength conversion element made of another material or having anotherstructure may be used. For example, potassium titanium photophate (KTP)or Mg:LiTaO₃ having a polarization reversal period structure may beused.

The range of the oscillation wavelength of the fundamental wave of thelaser light of the fiber laser 2103 is preferably 1028 to 1064 nm and1120 to 1200 nm in view of the relationship with the absorption spectrumof hemoglobin contained in blood if it is particularly restricted to thefield of medical devices.

As described above, according to the fifth embodiment, the powerconsumption of the entire device can be reduced by decreasing theoptical loss of the laser light. Further, the intermode competition canbe prevented by orthogonalizing the polarization directions of lightshaving different wavelengths, whereby a stable laser light output can beobtained. Furthermore, by making the Q-value of the laser cavityvariable based on the oscillation wavelength, inadvertent pulseoscillation of the laser light can be prevented while a reduction in theefficiency of the laser oscillator is suppressed. Therefore, thereliability of the device can be improved.

Since the laser light source device according to the above fifthembodiment has low power consumption at high luminance, it is useful inthe field of medical devices used at eye clinics and is also applicableas a display device such as a laser display.

The present invention can be summarized as follows from the aboverespective embodiments. Specifically, a laser light source deviceaccording to the present invention comprises a laser light source foremitting an excitation light; a laser cavity including a fiber whichcontains a laser-active substance and on which the excitation light fromthe laser light source is incident, a first fiber grating provided at aside of the fiber toward the laser light source and having a pluralityof reflection peaks, and a second fiber grating provided at a lightemission end of the fiber and having a plurality of reflection peaks; awavelength converter for converting a fundamental wave emitted from thelaser cavity into a harmonic wave; a reflection wavelength varying unitcapable of shifting the reflection wavelengths of the reflection peaksof the second fiber grating; and a controller for controlling theoscillation wavelength of the laser cavity by means of the reflectionwavelength varying unit and controlling a phase matching condition ofthe wavelength converter, intervals between adjacent reflection peaks ofthe first fiber grating being different from those between adjacentreflection peaks of the second fiber grating.

Since each of the first and second fiber gratings has a plurality ofreflection peaks and the intervals between the reflection peaks of thefirst fiber grating and those between the reflection peaks of the secondfiber grating are set to be different in the above laser light sourcedevice, the oscillation wavelength of the laser cavity can be switchedby shifting the reflection wavelengths of the reflection peaks of thesecond fiber grating.

Grating lengths for specifying the reflectances at the respectivereflection peaks of the second fiber grating are preferably set tosuccessively become longer in a decreasing direction of the oscillationwavelength of the laser cavity so as to prevent a reduction in theoscillation efficiency of the fiber resulting from a decrease of theoscillation wavelength of the laser cavity.

In this case, the oscillation efficiency of the fiber can be maintainedeven if the oscillation wavelength of the laser cavity is decreased.Particularly, a reduction in the oscillation efficiency of the fiberresulting from a decrease of the oscillation wavelength of the lasercavity is notable upon oscillating a light in a wavelength band of 1045to 1070 nm. Accordingly, in this case, the output of the light in thewavelength band of 1045 to 1070 nm can be improved.

It is preferable that the bandwidths of the reflection peaks of thefirst fiber grating are wider than those of the reflection peaks of thesecond fiber grating; and that the reflectances at the reflection peaksof the first fiber grating are larger than those at the reflection peaksof the second fiber grating.

In this case, accuracy required for a control of shifting thereflectances of the reflection peaks of the second fiber grating can bereduced since the bandwidths of the reflection peaks of the first fibergrating are wider than that of the reflection peaks of the second fibergrating, and the fundamental wave can be efficiently emitted from thesecond fiber grating since the reflectances at the reflection peaks ofthe first fiber grating are larger than those at the reflection peaks ofthe second fiber grating.

It is preferable that the bandwidths of the reflection peaks of thefirst fiber grating are 0.5 to 2 nm; and that the bandwidths of thereflection peaks of the second fiber grating are 0.2 nm or shorter.

In this case, the reflection peaks of the second fiber grating can bereliably brought into coincidence with those of the first fiber gratingupon shifting the reflectances of the reflection peaks of the secondfiber grating.

It is preferable that the reflectances at the reflection peaks of thefirst fiber grating are 95% or higher; and that those at the reflectionpeaks of the second fiber grating are 5 to 20%.

In this case, the laser light can be efficiently emitted while thebreakage of an excitation laser by the laser light from the laser cavityis prevented.

The controller preferably switches the oscillation wavelength of thelaser cavity by the shift of the reflection wavelengths at thereflection peaks of the second fiber grating by the wavelength converterand changes the phase matching condition of the wavelength converter inaccordance with the switched oscillation wavelength.

In this case, the fundamental wave can be efficiently wavelengthconverted into the harmonic wave since the phase matching condition ofthe wavelength converter is changed in conformity with the switch of theoscillation wavelength of the fundamental wave emitted from the lasercavity.

It is preferable that the wavelength converter includes a firstwavelength conversion element made of nonlinear crystal using anglephase matching and a first holder for holding the first wavelengthconversion element and setting an incident angle of the fundamental waveemitted from the laser cavity on the first wavelength conversion elementin accordance with the oscillation wavelength of the laser cavity so asto satisfy an angle phase matching condition of the first wavelengthconversion element; and that the controller controls the phase matchingcondition of the wavelength converter by means of the first holder.

In this case, the fundamental wave from the laser cavity can beefficiently wavelength converted into the harmonic wave in the case ofmaking the wavelength conversion element of the nonlinear crystal usingthe angle phase matching.

It is preferable that the wavelength converter includes a secondwavelength conversion element made of nonlinear crystal using anglephase matching and a second holder for holding the second wavelengthconversion element and setting an incident angle of the fundamental waveemitted from the first wavelength conversion element on the secondwavelength conversion element in accordance with the oscillationwavelength of the laser cavity so as to satisfy an angle phase matchingcondition of the second wavelength conversion element, the second holderbeing so arranged as to suppressed a change of an optical axis caused bythe first holder; and that the controller controls the phase matchingcondition of the wavelength converter by means of the first and secondholders.

In this case, the change of the optical axis caused upon the wavelengthconversion can be suppressed even if the wavelength conversion elementsare made of the nonlinear crystal using the angle phase matching.

It is preferable that the wavelength converter includes a wavelengthconversion element made of nonlinear crystal using temperature phasematching and a holder for holding the wavelength conversion element andsetting the temperature of the wavelength conversion element inaccordance with the oscillation wavelength of the laser cavity so as tosatisfy a temperature phase matching condition of the wavelengthconversion element; and that the controller controls the phase matchingcondition of the wavelength converter by means of the holder.

In this case, the fundamental wave from the laser cavity can beefficiently wavelength converted into the harmonic wave in the case ofmaking the wavelength conversion element of the nonlinear crystal usingthe temperature phase matching.

It is preferable that the wavelength converter includes a wavelengthconversion element made of nonlinear crystal using quasi phase matchingand having a polarization reversal period structure and a holder forholding the wavelength conversion element and causing the fundamentalwave emitted from the laser cavity to be incident in a polarizationreversal period area of the wavelength conversion element in accordancewith the oscillation wavelength of the laser cavity so as to satisfy aquasi phase matching condition of the wavelength conversion element; andthat the controller controls the phase matching condition of thewavelength converter by means of the holder.

In this case, the fundamental wave from the laser cavity can beefficiently wavelength converted into the harmonic wave in the case ofmaking the wavelength conversion element of the nonlinear crystal usingthe quasi phase matching.

It is preferable that the period of the polarization reversal periodstructure of the wavelength conversion element changes in a directionperpendicular to the incident direction of the fundamental wave emittedfrom the laser cavity; and that the holder includes a stage capable ofmoving the wavelength conversion element in such a direction as tochange the period of the polarization reversal period structure inaccordance with the oscillation wavelength of the laser cavity.

In this case, conversion efficiency from the fundamental wave into theharmonic wave can be improved since the period of the polarizationreversal structure of the wavelength conversion element can have anoptimal length.

The period of the polarization reversal structure of the wavelengthconversion element preferably changes along the incident direction ofthe fundamental wave emitted from the laser cavity.

In this case, the wavelength converter can be realized by a simpleconstruction since the fundamental wave can be incident in thepolarization reversal period area in accordance with the oscillationwavelength of the laser cavity without being accompanied by a movementof the wavelength conversion element.

It is preferable that there are two reflection peaks each of which hasan overlapping portion between the first and second fiber gratings; andthat the ranges of the respective reflection wavelengths of the tworeflection peaks are 1000 to 1090 nm and 1100 to 1180 nm.

In this case, the breakage of the laser cavity by the generation of anASE light can be prevented by causing the simultaneous generation of alight in the wavelength range of 1000 to 1090 nm even in the case ofgenerating a light in the wavelength range of 1100 to 1180 nm.

An image display device according to the present invention comprises theabove laser light source device; a projector control circuit including awavelength determining circuit connected to the controller of the laserlight source device for outputting a selection signal used to determinethe oscillation wavelength of the laser light source device to thecontroller, and a luminance signal judging circuit for judging theluminance of a video signal based on a luminance signal included in thevideo signal to be displayed by a laser light emitted from the laserlight source device; a video mode switching unit for instructing theoscillation wavelength of the laser light source device inputted by auser of the image display device to the wavelength determining circuit;and a power source control circuit for judging the type and remainingamount of a power source used by the image display device and outputtingthe judged type and remaining amount of the power source to thewavelength determining circuit.

In the above image display device, the oscillation wavelength of thelaser light emitted from the laser light source device can be switchedin accordance with the kind of the video signal and the status of use ofthe power source. Thus, there can be realized an image display devicehaving a wide color reproduction area at high luminance, a high imagequality and low power consumption.

A microscope device according to the present invention comprises theabove laser light source device, and a microscope unit for enabling theobservation of the light emission of a fluorescent sample caused by theexcitation of the fluorescent sample in response to the irradiation of alaser light emitted from the laser light source device.

In the above microscope device, a plurality of fluorescent samples canbe efficiently excited since the oscillation wavelength of the laserlight emitted from the laser light source device can be switcheddepending on the fluorescent sample.

Another laser light source device according to the present inventioncomprises a laser oscillator for emitting at least two fundamentalwaves; a laser light irradiator adapted to convert the at least twofundamental waves emitted from the laser oscillator respectively intoharmonic waves and capable of irradiating the converted harmonic waves;and a fiber unit disposed between the laser oscillator and the laserlight irradiator for transmitting the fundamental waves emitted from thelaser oscillator to the laser light irradiator, wherein the laseroscillator includes a laser light source for emitting an excitationlight; and a laser cavity including a fiber which contains alaser-active substance and on which the excitation light from the laserlight source is incident, at least two first fiber gratings provided ata side of the fiber toward the laser light source, and at least twosecond fiber gratings provided at a light emission end of the fiber andhaving a one-to-one correspondence with the at least two second fibergratings.

In the above laser light source device, at least two fundamental wavesare emitted from the laser oscillator, transmitted to the laser lightirradiator via the fiber unit, and respectively wavelength convertedinto harmonic waves by the laser light irradiator. Thus, at least twolaser lights having different wavelengths can be more efficientlyoscillated and irradiated than before.

It is preferable that the laser cavity further includes a lightbranching unit for branching the light emitted from the fiber inaccordance with polarization directions; and that any one of branchedlights branched by the light branching unit is incident on each secondfiber grating.

In this case, the intermode competition occurring at the time ofmultiwavelength oscillation can be suppressed since the fundamental waveof a different wavelength is oscillated in each polarization directionof the light emitted from the fiber.

Each second fiber grating is preferably constructed such that thereflection wavelength thereof is shiftable so as to make a Q-value ofthe cavity formed between the second fiber grating and the correspondingfirst fiber grating variable.

In this case, the Q-value of the cavity formed between the first andsecond fiber gratings can be changed by shifting the reflectionwavelength of the second fiber grating. Thus, the Q-value of the cavityof the fundamental wave unnecessary for the light irradiation by thelaser light irradiator can be decreased to stop the oscillation of thecavity.

It is preferable that the laser light irradiator includes at least twowavelength conversion elements for converting the fundamental waveemitted from the laser cavity into harmonic waves and optical membersfor reflecting parts of the harmonic waves emitted from the wavelengthconversion elements; that the harmonic waves reflected by the opticalmembers are fed back to the laser oscillator by the fiber unit; and thatthe laser oscillator changes the oscillation wavelength of the lasercavity based on the output intensities of the fed-back harmonic waves.

In this case, the outputs of the harmonic waves emitted from the laserlight irradiator can be stabilized since the oscillation wavelength ofthe laser cavity is changed based on the output intensities of theharmonic waves wavelength converted by the laser light irradiator.

The reflectances of the optical members are preferably 1 to 10%, and theoptical members are preferably dielectric multilayer films arranged onthe light emission surfaces of the wavelength conversion elements.

In this case, the laser oscillator can accurately detect the outputintensities of the harmonic waves wavelength converted by the laserlight irradiator.

The laser light irradiator preferably further includes a light selectorfor causing the laser light irradiator to selectively irradiate eitherone of the lights emitted from the at least two wavelength conversionelements.

In this case, the wavelength of the laser light emitted from the laserlight irradiator can be switched.

The fiber unit preferably includes double-clad fibers.

In this case, the transmission of the fundamental wave from the laseroscillator to the laser light irradiator and that of the harmonic wavesfrom the laser light irradiator to the laser oscillator can beefficiently performed.

The second fiber grating preferably has the reflection wavelengththereof shifted by a control of the temperature thereof or theapplication of a tensile stress thereto.

In this case, the reflection wavelength of the second fiber grating canbe accurately shifted.

The laser oscillator preferably further includes a detector fordetecting the output intensities of the harmonic waves fed back by thefiber unit, and a controller for controlling shift amounts of thereflection wavelengths of the second fiber gratings based on the outputintensities of the harmonic waves detected by the detector.

In this case, the oscillation wavelength of the laser cavity can bechanged in response to the output intensities of the harmonic waveswavelength converted by the laser light irradiator since the outputintensities of the harmonic waves fed back from the laser lightirradiator are detected and the shift amounts of the reflectionwavelengths of the second fiber gratings are controlled based on thedetected output intensities of the harmonic waves. As a result, theoutputs of the harmonic waves emitted from the laser light irradiatorcan be stabilized.

The detector is preferably a light receiving element arranged in thevicinity of a connection point of the laser cavity and the fiber unitfor receiving leak lights of the harmonic waves from the connectionpoint.

In this case, the output intensities of the harmonic waves fed back fromthe laser light irradiator can be accurately grasped.

It is preferable that the laser oscillator further includes a switchunit to which a turn-on instruction to turn the laser light source on isinputted by a user of the laser light source device; that the controllercauses the laser light source to be turned on in accordance with theturn-on instruction inputted via the switch unit and controls the shiftamounts of the reflection wavelengths of the second fiber gratings tostabilize the output intensities of the harmonic waves detected by thedetector; and the laser light irradiator further includes a switch forenabling the harmonic waves emitted from the wavelength conversionelements to be irradiated from the laser light irradiator after theoutput intensities of the harmonic waves are stabilized.

In this case, it is no longer necessary to constantly operate the laserlight source since the light irradiation of the laser light irradiatoris possible after the outputs of the harmonic waves wavelength convertedby the laser light irradiator are stabilized. As a result, powerconsumption by the laser light source can be reduced.

The controller preferably causes the fundamental wave emitted from thelaser cavity when the phase matching conditions of the wavelengthconversion elements come to be not satisfied by the control of the shiftamounts of the reflection wavelengths of the second fiber gratings to beemitted from the wavelength conversion elements without being wavelengthconverted by the wavelength conversion elements.

In this case, the output of the fundamental wave emitted from the laserlight irradiator can be increased.

The reflection wavelength ranges of the at least two fundamental wavesemitted from the laser cavity are preferably from 1000 to 1100 nm andfrom 1100 to 1200 nm.

In this case, the breakage of the laser cavity by the generation of anASE light can be prevented by simultaneously generating a light in thewavelength band of 1000 to 1100 nm even in the case of generating alight in the wavelength range of 1100 to 1200 nm.

It is preferable that the wavelength conversion elements are composed ofMg:LiNbO₃ crystals using the quasi phase matching; and that thewavelength bandwidths of the fundamental waves emitted from the lasercavity is 0.06 nm or shorter.

In this case, wavelength conversion efficiency from the fundamentalwaves into the harmonic waves can be 90% or higher of a maximumwavelength conversion efficiency if the wavelength conversion elementsare composed of Mg:LiNbO₃ crystals using the quasi phase matching.

This application is based on patent application No. 2006-172138 filed inJapan, the contents of which are hereby incorporated by references.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within metesand bounds of the claims, or equivalence of such metes and bounds aretherefore intended to embraced by the claims.

1. A laser light source device, comprising: a laser light source foremitting an excitation light; a laser cavity including a fiber whichcontains a laser-active substance and on which the excitation light fromthe laser light source is incident, a first fiber grating provided at aside of the fiber toward the laser light source and having a pluralityof reflection peaks, and a second fiber grating provided at a lightemission end of the fiber and having a plurality of reflection peaks; awavelength converter for converting a fundamental wave emitted from thelaser cavity into a harmonic wave; a reflection wavelength varying unitcapable of shifting the reflection wavelengths of the reflection peaksof the second fiber grating; and a controller for controlling theoscillation wavelength of the laser cavity by means of the reflectionwavelength varying unit and controlling a phase matching condition ofthe wavelength converter, wherein intervals between adjacent reflectionpeaks of the first fiber grating are different from those betweenadjacent reflection peaks of the second fiber grating.
 2. A laser lightsource device according to claim 1, wherein grating lengths forspecifying the reflectances at the respective reflection peaks of thesecond fiber grating are set to successively become longer in adecreasing direction of the oscillation wavelength of the laser cavityso as to prevent a reduction in the oscillation efficiency of the fiberresulting from a decrease of the oscillation wavelength of the lasercavity.
 3. A laser light source device according to claim 1, wherein:the bandwidths of the reflection peaks of the first fiber grating arewider than those of the reflection peaks of the second fiber grating;and the reflectances at the reflection peaks of the first fiber gratingare larger than those at the reflection peaks of the second fibergrating.
 4. A laser light source device according to claim 3, wherein:the bandwidths of the reflection peaks of the first fiber grating are0.5 to 2 nm; and the bandwidths of the reflection peaks of the secondfiber grating are 0.2 nm or shorter.
 5. A laser light source deviceaccording to claim 3, wherein: the reflectances at the reflection peaksof the first fiber grating are 95% or higher; and the reflectances atthe reflection peaks of the second fiber grating are 5 to 20%.
 6. Alaser light source device according to claim 1, wherein the controllerswitches the oscillation wavelength of the laser cavity by the shift ofthe reflection wavelengths at the reflection peaks of the second fibergrating by the wavelength converter and changes the phase matchingcondition of the wavelength converter in accordance with the switchedoscillation wavelength.
 7. A laser light source device according toclaim 6, wherein: the wavelength converter includes a first wavelengthconversion element made of nonlinear crystal using angle phase matchingand a first holder for holding the first wavelength conversion elementand setting an incident angle of the fundamental wave emitted from thelaser cavity on the first wavelength conversion element in accordancewith the oscillation wavelength of the laser cavity so as to satisfy anangle phase matching condition of the first wavelength conversionelement; and the controller controls the phase matching condition of thewavelength converter by means of the first holder.
 8. A laser lightsource device according to claim 7, wherein: the wavelength converterfurther includes a second wavelength conversion element made ofnonlinear crystal using the angle phase matching and a second holder forholding the second wavelength conversion element and setting an incidentangle of the fundamental wave emitted from the first wavelengthconversion element on the second wavelength conversion element inaccordance with the oscillation wavelength of the laser cavity so as tosatisfy an angle phase matching condition of the second wavelengthconversion element, the second holder being so arranged as to suppress achange of an optical axis caused by the first holder; and the controllercontrols the phase matching condition of the wavelength converter bymeans of the first and second holders.
 9. A laser light source deviceaccording to claim 6, wherein: the wavelength converter includes awavelength conversion element made of nonlinear crystal usingtemperature phase matching and a holder for holding the wavelengthconversion element and setting the temperature of the wavelengthconversion element in accordance with the oscillation wavelength of thelaser cavity so as to satisfy a temperature phase matching condition ofthe wavelength conversion element; and the controller controls the phasematching condition of the wavelength converter by means of the holder.10. A laser light source device according to claim 6, wherein: thewavelength converter includes a wavelength conversion element made ofnonlinear crystal using quasi phase matching and having a polarizationreversal period structure and a holder for holding the wavelengthconversion element and causing the fundamental wave emitted from thelaser cavity to be incident in a polarization reversal period area ofthe wavelength conversion element in accordance with the oscillationwavelength of the laser cavity so as to satisfy a quasi phase matchingcondition of the wavelength conversion element; and the controllercontrols the phase matching condition of the wavelength converter bymeans of the holder.
 11. A laser light source device according to claim10, wherein: the period of the polarization reversal period structure ofthe wavelength conversion element changes in a direction perpendicularto the incident direction of the fundamental wave emitted from the lasercavity; and the holder includes a stage capable of moving the wavelengthconversion element in such a direction as to change the period of thepolarization reversal period structure in accordance with theoscillation wavelength of the laser cavity.
 12. A laser light sourcedevice according to claim 10, wherein the period of the polarizationreversal structure of the wavelength conversion element changes alongthe incident direction of the fundamental wave emitted from the lasercavity.
 13. A laser light source device according to claim 1, whereinthere are two reflection peaks each of which has an overlapping portionbetween the first and second fiber gratings.
 14. A laser light sourcedevice according to claim 13, wherein the ranges of the respectivereflection wavelengths of the two reflection peaks are 1000 to 1090 nmand 1100 to 1180 nm.
 15. An image display device, comprising: a laserlight source device according to claim 1; a projector control circuitincluding a wavelength determining circuit connected to the controllerof the laser light source device for outputting a selection signal usedto determine the oscillation wavelength of the laser light source deviceto the controller, and a luminance signal judging circuit for judgingthe luminance of a video signal based on a luminance signal included inthe video signal to be displayed by a laser light emitted from the laserlight source device; a video mode switching unit for instructing theoscillation wavelength of the laser light source device inputted by auser of the image display device to the wavelength determining circuit;and a power source control circuit for judging the type and remainingamount of a power source used by the image display device and outputtingthe judged type and remaining amount of the power source to thewavelength determining circuit.
 16. A microscope device, comprising: alaser light source device according to claim 1; and a microscope unitfor enabling the observation of the light emission of a fluorescentsample caused by the excitation of the fluorescent sample in response tothe irradiation of a laser light emitted from the laser light sourcedevice.
 17. A laser light source device, comprising: a laser oscillatorfor emitting at least two fundamental waves; a laser light irradiatoradapted to convert the at least two fundamental waves emitted from thelaser oscillator respectively into harmonic waves and capable ofirradiating the converted harmonic waves; and a fiber unit disposedbetween the laser oscillator and the laser light irradiator fortransmitting the fundamental waves emitted from the laser oscillator tothe laser light irradiator, wherein the laser oscillator includes: alaser light source for emitting an excitation light; and a laser cavityincluding a fiber which contains a laser-active substance and on whichthe excitation light from the laser light source is incident, at leasttwo first fiber gratings provided at a side of the fiber toward thelaser light source, and at least two second fiber gratings provided at alight emission end of the fiber and having a one-to-one correspondencewith the at least two second fiber gratings.
 18. A laser light sourcedevice according to claim 17, wherein: the laser cavity further includesa light branching unit for branching the light emitted from the fiber inaccordance with polarization directions; and any one of branched lightsbranched by the light branching unit is incident on each second fibergrating.
 19. A laser light source device according to claim 18, whereineach second fiber grating is constructed such that the reflectionwavelength thereof is shiftable so as to make a Q-value of the cavityformed between the second fiber grating and the corresponding firstfiber grating variable.
 20. A laser light source device according toclaim 19, wherein: the laser light irradiator includes at least twowavelength conversion elements for converting the fundamental waveemitted from the laser cavity into harmonic waves and optical membersfor reflecting parts of the harmonic waves emitted from the wavelengthconversion elements; the harmonic waves reflected by the optical membersare fed back to the laser oscillator by the fiber unit; and the laseroscillator changes the oscillation wavelength of the laser cavity basedon the output intensities of the fed-back harmonic waves.
 21. A laserlight source device according to claim 20, wherein the reflectances ofthe optical members are 1 to 10%.
 22. A laser light source deviceaccording to claim 20, wherein the optical members are dielectricmultilayer films arranged on the light emission surfaces of thewavelength conversion elements.
 23. A laser light source deviceaccording to claim 20, wherein the laser light irradiator furtherincludes a light selector for causing the laser light irradiator toselectively irradiate either one of the lights emitted from the at leasttwo wavelength conversion elements.
 24. A laser light source deviceaccording to claim 17, wherein the fiber unit includes double-cladfibers.
 25. A laser light source device according to claim 20, whereinthe second fiber gratings have the reflection wavelengths thereofshifted by a control of the temperature thereof or the application of atensile stress thereto.
 26. A laser light source device according toclaim 20, wherein the laser oscillator further includes: a detector fordetecting the output intensities of the harmonic waves fed back by thefiber unit; and a controller for controlling shift amounts of thereflection wavelengths of the second fiber gratings based on the outputintensities of the harmonic waves detected by the detector.
 27. A laserlight source device according to claim 26, wherein the detector is alight receiving element arranged in the vicinity of a connection pointof the laser cavity and the fiber unit for receiving leak lights of theharmonic waves from the connection point.
 28. A laser light sourcedevice according to claim 26, wherein: the laser oscillator furtherincludes a switch unit to which a turn-on instruction to turn the laserlight source on is inputted by a user of the laser light source device;the controller causes the laser light source to be turned on inaccordance with the turn-on instruction inputted via the switch unit andcontrols the shift amounts of the reflection wavelengths of the secondfiber gratings to stabilize the output intensities of the harmonic wavesdetected by the detector; and the laser light irradiator furtherincludes a switch for enabling the harmonic waves emitted from thewavelength conversion elements to be irradiated from the laser lightirradiator after the output intensities of the harmonic waves arestabilized.
 29. A laser light source device according to claim 26,wherein the controller causes the fundamental wave emitted from thelaser cavity when the phase matching conditions of the wavelengthconversion elements come to be not satisfied by the control of the shiftamounts of the reflection wavelengths of the second fiber gratings to beemitted from the wavelength conversion elements without being wavelengthconverted by the wavelength conversion elements.
 30. A laser lightsource device according to claim 17, wherein the reflection wavelengthranges of the at least two fundamental waves emitted from the lasercavity are from 1000 to 1100 nm and from 1100 to 1200 nm.
 31. A laserlight source device according to claim 20, wherein: the wavelengthconversion elements are composed of Mg:LiNbO₃ crystals using the quasiphase matching; and the wavelength bandwidths of the fundamental wavesemitted from the laser cavity are 0.06 nm or shorter.